Preheat oven to 350°F (180°C). Spray 2 (8-inch) round cake pans with baking spray with flour. Line bottom of pans with parchment paper.: As the oven warms, you will notice a faint, clean heat that signals readiness; the sound of the preheat clicking off is my cue to move quickly. Preparing the pans with spray and parchment prevents sticking and makes for clean release, which preserves the edges of your layers. If you skip lining, the cakes may tear when removed, so take the extra minute. When you press the parchment into the pan it should lay flat, and the prepared pans should look lightly dusted with flour so batter spreads evenly.
In a medium bowl, whisk together flour, cinnamon, baking powder, salt, baking soda, allspice, nutmeg, and ginger.: You'll want the spices evenly dispersed, so whisk until the flour takes on a uniform, warm hue and there are no clumps. The scent will shift from neutral flour to a fragrant spiced mixture, which is a clear visual and olfactory signal you are ready for the wet ingredients. Proper whisking prevents pockets of leavening or spice from concentrating, which can cause uneven rise or hotspots of flavor. If you notice lumps, press them through a fine mesh strainer or sift briefly to ensure smooth incorporation.
In a large bowl, whisk together buttermilk, applesauce, oil, and vanilla; stir in sugars. Add flour mixture, and whisk until just combined. Divide batter between prepared pans, smoothing tops.: The wet bowl will look glossy and pourable, and when you stir in the granulated sugar and dark brown sugar , the mixture will soften and glisten. Folding in the dry mixture just until combined preserves a tender crumb, so stop when the streaks of flour disappear. The batter should hold a slight sheen and flow slowly when divided into the pans; over mixing introduces gluten and makes the cake dense. If you see vigorous streaks of flour, gently fold a few more strokes rather than vigorous whisking.
Bake until a wooden pick inserted in center comes out with a few moist crumbs, 25 to 30 minutes. Let cool in pans for 10 minutes. Remove from pans, and let cool completely on wire racks.: While baking, the kitchen will fill with a warm spice aroma and you may hear a quiet settling as the cake sets. Check at the lower end of the time window; insert a pick and look for a few moist crumbs clinging to it, not raw batter. Cooling in the pans for 10 minutes lets the crumb stabilize, which helps when you invert them onto racks; rush this step and the layers might crack. Once on the racks, the tops should feel springy and slightly warm, and the edges should have pulled just a hair away from the pan.
Level cooled cake layers, if desired. Place 1 cake layer, cut side up, on a serving plate. Spread 1 1/2 cups (306 grams) Vanilla Bean Buttercream onto cake layer. Top with remaining cake layer. Spread remaining Vanilla Bean Buttercream on top and sides of cake, creating a slight border around top edge. Refrigerate for at least 30 minutes.: When the layers are fully cool, leveling ensures even stacking and a professional silhouette. The buttercream should be smooth and spreadable, not runny; it will feel creamy and slightly tacky as you spread. Creating a shallow border prevents the Spiced Apple Topping from spilling over, and chilling the cake helps the frosting firm so the topping sits neatly. A common mistake is applying frosting while the cake is warm, which causes melting and a messy finish.
Just before serving, spoon Spiced Apple Topping on top of cake within border. Cover and refrigerate for up to 3 days.: Add the topping shortly before guests arrive so it retains a glossy, fresh look and lively texture. The topping should shimmer and offer small audible spoon sounds as you portion it, a contrast to the soft cake beneath. Refrigeration will keep the cake stable, and storing in an airtight container helps maintain moisture; however, prolonged refrigeration can slightly mute the spices. If the topping sits too long on the cake, it can release moisture and soften the border, so serve within a day if you want the most distinct textures.