Preheat the oven to 350 F and spray two 9-inch x 2-inch cake pans very well with floured cooking spray, or grease and flour the pans; set aside.: When you preheat to 350 F , you will feel the kitchen warm and smell the faint metallic note of the heating elements if your oven is new. Spraying and flouring the pans ensures the cakes release cleanly, so the edges will pull away slightly from the pan and the top will not stick. If you skip proper pan prep, the cake can tear when you turn it out, so take the extra minute to coat the interior evenly, tapping out excess flour. A common mistake is using too much flour which can create a thin dry crust, so shake out any large excess. Visually, the pans should look evenly coated without clumps of flour.
Sift the flour, sugar, cocoa, baking soda, baking powder, and salt into a medium bowl or the bowl; set aside.: The sound of the sifter moving and the powder falling into the bowl feels almost meditative, and sifting helps aerate the dry mix so the crumb is light rather than dense. This step prevents lumps of cocoa powder or baking powder from creating uneven pockets in the cake. If you rush this, you might find small clumps in the batter that do not incorporate, causing tiny gritty spots in the finished cake. Make sure the bowl is dry, because any moisture will create immediate clumps in your dry ingredients.
To a large bowl or the bowl of a stand mixer, add the buttermilk, oil, eggs, vanilla, and beat with a handheld electric mixer on high power or with the paddle attachment of a stand mixer on high power, about 90 seconds.: As you beat these wet ingredients, you will hear a steady whirl and notice the mixture lighten slightly in color and become cohesive. The buttermilk adds tang and the oil provides silkiness; whisking at high speed encourages emulsion, which helps the batter hold air. Overbeating once dry ingredients are in can develop the gluten too much, so this timing is important. If the mixture looks separated, stop and scrape the sides, then continue for a few more seconds until smooth. A common error is adding cold eggs which can make the batter seize visually, so bring them to room temperature.
With the mixer on low speed, slowly add the dry ingredients and mix until combined, about 1 to 2 minutes.: The quiet hum at low speed allows the dry mix to incorporate gently without flour clouds erupting. You will see the batter thicken and darken as the cocoa powder disperses. Mixing too long at this stage can yield a tight, chewy crumb because of gluten development, so stop as soon as you no longer see streaks of dry ingredients. If you notice clumps clinging to the paddle, scrape the bowl and fold them in by hand to avoid overmixing. The batter should look smooth but not glossy.
Add the coffee and mix to just to combine; the batter will be very thin.: When the warm coffee hits the batter, you will see it loosen dramatically and steam may rise slightly if the coffee is very warm. The coffee deepens and sharpens the chocolate notes, creating a pronounced cocoa flavor. Stir only until the liquid is incorporated, because overmixing will change the texture, and adding very hot liquid can cook the eggs, so make sure the coffee is warm rather than boiling. The batter should pour easily from a spoon and feel silky on the back of your hand.
Pour batter into prepared pans and bake for about 35 to 40 minutes or until a cake tester comes out clean or with a few moist crumbs but no batter. Start checking cakes at 25 minutes because all ovens vary.: As the cake bakes, you will notice the aroma deepen into a rich chocolate scent and the surface become matte and slightly springy. The edges might pull away a touch from the pan, another sign of readiness. Use a tester inserted into the center; a few moist crumbs are fine, raw batter is not. Opening the oven too often can cause uneven rise, so check starting at 25 minutes and then in short intervals. If a tester shows wet batter, return the cake and continue monitoring closely to avoid overbaking which causes dryness.
Cool cakes in the pans for about 30 minutes before turning them out onto a cooling rack to cool completely.: Letting the cakes rest in the pan allows the structure to set and reduces the risk of collapse. You will notice steam slow down and the cake shrink slightly from the pan sides. After about 30 minutes, run a thin knife around the edge if needed and invert gently onto a cooling rack . If you try to unmold too early, the cake can fall apart; too late and the exterior might be sticky and tear. The final cooled cakes should feel springy and be completely cool before frosting to avoid melting the icing.
To a large bowl or the bowl of a stand mixer, beat the butter and cream cheese with a handheld electric mixer on high power or with the paddle attachment of a stand mixer on high power until light and fluffy, about 2 to 3 minutes.: Whipping butter and cream cheese incorporates air so the frosting becomes light rather than dense. You will see the color shift to a paler tone and the texture become silky. Make sure both are fully softened, because cold lumps will not smooth out and will leave small pockets in the frosting. If the mixture appears curdled, keep beating and scraping the bowl; it should come together. Overbeating for too long can melt the mixture, so stop once it is airy and smooth.
Reduce the speed to low and slowly add the cocoa powder, vanilla, salt, and beat until combined.: Adding the sifted cocoa powder at low speed keeps the frosting from puffing with dry dust, and you will notice the aroma deepen as it is incorporated. The vanilla and pinch of salt sharpen the overall balance. If you add the cocoa too quickly you may create a cloud of dry powder, which is messy and can lead to uneven mixing. The texture should become uniformly chocolate and paste like before the confectioners' sugar goes in.
With mixer on low speed, gradually add the confectioners’ sugar, one cup at a time. The frosting will be incredibly thick, almost too thick to mix; this is ok.: As you add the confectioners' sugar , the frosting will thicken and resist the paddle; that density is normal and indicates a stable frosting that will hold shape. Work slowly so the sugar blends and avoid a gritty texture by sifting if needed. If the mixer strains, pause and scrape the bowl, then continue slowly. Rushing to add all the sugar at once can create dust and uneven pockets that are hard to smooth out.
Add the cream as necessary to create a spreadable consistency (I used about 4 1/2 tablespoons). The frosting should be quite thick and it also will thicken more after the cake has been refrigerated.: Adding cream loosens the frosting to a workable state; you want it spreadable but still substantial enough to hold decorative edges. Start with less than you think you need and add more to reach the desired texture. Keep in mind refrigeration will firm it further, so aim for slightly softer than your final intent. If you add too much liquid, you can correct by adding a touch more confectioners' sugar to thicken again.
To frost the cake, place 1 layer, flat side up, on a flat plate or cake pedestal. With a knife or offset spatula, spread the top with frosting.: Placing the first layer flat side up gives a stable base for even spreading, and the moment you touch the surface with an offset spatula you will feel the thick frosting glide and leave soft peaks. Apply in the center and push outward to avoid pulling crumbs into the icing. If crumbs appear, do a thin crumb coat and chill briefly before the final layer. A common misstep is applying too much pressure which can tear the crumb, so use gentle, even passes.
Place the second layer on top, flat side up, and spread the frosting evenly on the top and sides of the cake.: Setting the second layer completes the silhouette and you will notice the cake settle slightly as the weight redistributes. Use sweeping motions to smooth the top and turn small amounts of frosting up the sides in steadier strokes for a clean finish. If the sides look uneven, chill the cake for a short period and then use a bench scraper for a polished edge. Avoid dragging the spatula repeatedly in the same spot which can create ridges.
Evenly sprinkle the top of the cake with mini chocolate chips.: As you scatter the mini semi-sweet chocolate chips , they add playful texture and visual contrast, landing like tiny roasted pebbles on the silky surface. Press them in lightly so they adhere, but not so firmly that they sink. Space them evenly for a balanced look, and if any melt slightly from residual warmth, wait for it to set in the refrigerator so they maintain their shape.
Refrigerate the cake for 1 to 2 hours before serving.: Chilling the assembled cake allows the frosting to firm and the flavors to meld, which makes slicing neater and flavor deeper. You will notice the frosting lose some sheen and become denser to the touch after refrigeration. If you cut it too soon, the frosting will smear and slices will be messy, so patience here rewards you with tidy, attractive pieces. Remove from the fridge a short time before serving so it softens slightly for the best mouthfeel.
Optionally, before serving, drizzle with melted chocolate chips. Cake will keep airtight in the fridge for up to 5 days.: A warm drizzle of melted semi-sweet chocolate chips adds gloss and a slight snap once it sets, creating contrast against the creamy frosting. Pour in a slow, steady stream to create ribbons, or use a fork for a fine web. If the drizzle cools too quickly and clumps, gently rewarm in short bursts. Store airtight in the fridge to preserve freshness, and bring to room temperature briefly before serving for the best texture and flavor.