Preheat the oven to 350 degrees (F). Line a 12-cup cupcake/muffin tin with cupcake liners and lightly spray the liners with non-stick spray (optional but it does help them peel right off).: The warm oven smell is the first signal that baking is starting, and you should notice the air filling with a faint toasty note as it reaches 350 degrees (F) . Properly prepared liners ensure the cupcakes release cleanly, and a light spray helps prevent tearing when peeling. Use this time to measure ingredients, because once the mixing starts it moves quickly. A common error is skipping liners which makes removal messy, so line and spray ahead of time.
Melt the oil, butter, and chocolate together in the microwave, heating in 30 second increments, stirring between increments. You can also melt the oil, butter, and chocolate over very low heat on the stove top, but I find the microwave to be much easier. Whisk mixture until completely smooth and set aside to cool.: As you melt, observe how the mixture becomes glossy and velvety, with small chocolate ribbons dissolving into the melted unsalted butter and canola oil . Stirring between short bursts prevents scorching and promotes a satiny emulsion. Let it rest until it is warm rather than hot so it will blend with the eggs without cooking them. Avoid overheating which can cause graininess or separation; if that happens, whisk vigorously to reincorporate or rewarm gently.
In a medium-sized bowl combine the flour, baking soda, baking powder, cocoa powder, and salt; stir together until thoroughly combined; set aside.: When dry ingredients are mixed, inhale the cocoa aroma and note the cocoa dust settling; this blend ensures even distribution of leaveners and salt throughout the batter. Sifting or whisking helps remove lumps and creates a uniform mixture. This step prevents pockets of rising agent which could make uneven domes. Mixing well now saves you from inconsistent crumb later.
In a large bowl whisk together the eggs, yolk, sugar and vanilla extract; beat until smooth.: Whisking these eggs and granulated sugar creates a pale, slightly viscous base, and you will see the mixture lighten as air is incorporated. This aeration contributes to a tender crumb with gentle lift. Ensure your eggs are at room temperature for better emulsification; cold eggs inhibit volume. Over beating can make the batter too airy, which can collapse during baking.
Add the cooled oil/butter/chocolate mixture and whisk until smooth. Add half of the flour mixture, then half of the sour cream. Repeat the process until everything is added, and be sure to mix until JUST combined. Quickly stir in the hot coffee - it's important not to over mix, here! Just stir until everything is evenly combined.: When the melted chocolate hits the egg base you will see a glossy ribbon forming, and the batter will smell profoundly of chocolate. Alternating dry ingredients with sour cream helps maintain tenderness and prevents overworking gluten. Adding the hot coffee at the end magnifies chocolate depth and thins the batter slightly, creating a silky consistency. The crucial mistake to avoid is over mixing which tightens the crumb, so stop as soon as the streaks disappear.
Divide the batter among the 12 liners in your prepared pan. Bake for 16-18 minutes, or until a toothpick inserted in the center comes out clean. Cool completely before frosting.: As the cupcakes bake you will notice the kitchen filling with a warm, roasted chocolate scent and the cupcakes doming gently. Visual cues include set edges and a springy top when touched. Removing them too early yields a gummy center, while overbaking dries the crumb. Cool fully so the frosting does not melt into the cupcakes, which keeps the final presentation neat.
Sift together the confectioners sugar and cocoa powder, whisking well to ensure it's bump and lump free!: Sifting creates a feather light base for the frosting, and when you whisk them together you should see a fine, even powder with a deep chocolate color. This prevents gritty frosting and ensures the cocoa integrates smoothly with softened butter . If lumps remain, press them through a fine sieve. A common mistake is skipping sifting and ending up with gritty texture.
Using a handheld mixer OR a stand mixer fitted with the paddle attachment, beat the butter on medium-high speed until creamy; about 2 minutes. Reduce speed to low and slowly add the sifted sugar/cocoa powder, alternating with the sour cream and milk, then add the vanilla extract and salt. Once all of the ingredients have been added, increase speed to medium-high and beat the frosting until fluffy, about 2 minutes.: When properly beaten the butter will become pale and pillowy, and as you add the sifted sugar/cocoa blend it will transform into a glossy, pipe friendly frosting. Alternating in sour cream and milk ensures a silky texture without becoming runny. Beat at the end to introduce air for lightness. Avoid adding too much liquid at once, which can collapse the structure.
Frost cooled cupcakes as desired, then top with chocolate drizzle, sprinkles, and a toasted marshmallow.: The contrast between cool, fluffy buttercream and warm, toasted marshmallow is delightful; you should hear a soft sigh when the marshmallow skin cracks and reveal a gooey inside. Apply the frosting with a spatula or piping bag for different looks, then spoon the glaze and add holiday sprinkles quickly before it sets. A frequent error is placing marshmallows too early which makes them soggy, so toast as the final step.
Cupcakes stay fresh, stored in the fridge, in an airtight container, for up to 3 days. Bring to room temperature before serving.: Chilling firms the buttercream and helps set the glaze, and when you bring cupcakes back to room temperature the frosting softens and flavors bloom. Store them in a single layer to avoid damage to decorations. Do not leave them unrefrigerated for extended periods if the kitchen is warm, as the frosting can become unstable.
Place chocolate in a medium-sized heat-proof bowl; set aside.: Preparing the bittersweet chocolate in a bowl readies it for a hot cream pour to create a smooth glaze. Chop chocolate uniformly so it melts at the same rate. Placing it aside allows time for the cream to reach the correct temperature. Skipping this prep can lead to uneven melting.
In a small saucepan over medium heat, warm the heavy cream until very hot, but not boiling. Pour the hot cream over the chopped chocolate and set it aside for 2-3 minutes. Whisk the cream and chocolate completely smooth; whisk in vanilla. Allow the chocolate to cool for about 5 minutes before spooning the on the top of each frosted cupcake. Top the wet chocolate with sprinkles and a toasted marshmallow.: When you pour hot heavy whipping cream over the chopped chocolate, watch the surface become glossy as the heat melts the chocolate into a silky glaze. Waiting a few minutes before whisking prevents seizing, and whisking yields a shiny, smooth emulsion. Adding vanilla extract brightens aroma. If the glaze is too hot it can melt the frosting, so cool slightly before spooning.
Place the marshmallows on a large baking sheet and set aside. Position one of your oven racks right below the broiler; preheat your oven to broil. Once the oven reaches the broiler setting, place marshmallows in the oven to toast.: Adding sprinkles to the wet glaze ensures they adhere and adds festive color. Toasted marshmallow placed after the glaze creates a striking focal point and delivers a warm bite above the cool frosting. Work quickly so the glaze remains tacky; if it sets too fast you may need to rewarm slightly.
Warning: The marshmallows toast almost immediately - so stay right near the oven! Once browned, quickly remove the pan from the oven and let cool for a minute or two. Using a spatula remove the marshmallows from the pan and place one on top of each frosted cupcakes.: The broiler will brown the marshmallows in moments, producing a crackly, caramelized surface and a warm, slightly smoky aroma. Keep the oven door open a touch and stand watch because they can go from golden to burnt almost instantly. A safer alternative is a small handheld torch for direct control.
Warning: The marshmallows toast almost immediately - so stay right near the oven! Once browned, quickly remove the pan from the oven and let cool for a minute or two. Using a spatula remove the marshmallows from the pan and place one on top of each frosted cupcakes : Listen for the faint rapid sizzling as the outer layer chars, and you will see a golden brown that signals readiness. Cool briefly so the marshmallows set but remain pliable for placing. A common mistake is leaving them under the broiler too long, producing bitter burnt edges, so watch closely.