Chocolate Mousse Cake Recipe

Prepare

Preheat oven to 350°F (175°C). Grease and line two 8-inch round cake pans (or one 9-inch) with parchment; grease parchment and dust with cocoa powder.

Dry ingredients

In a large bowl, whisk together flour, sugar, cocoa powder, baking powder, baking soda, and salt until well combined.

Wet ingredients

In a separate bowl, whisk eggs, buttermilk, oil, and vanilla until smooth.

Combine

Pour wet mixture into dry ingredients and whisk until just combined. Gradually whisk in hot water until batter is smooth and slightly thin. Do not overmix.

Bake

Divide batter evenly between prepared pans. Bake 20–25 minutes (8-inch) or until a toothpick inserted into center comes out with a few moist crumbs. Cool in pans 10 minutes, then invert onto racks and cool completely. If cake is domed, level tops once cool.

Bloom gelatin & melt chocolate

Sprinkle 1 tsp gelatin over 2 tbsp cold water in a small bowl; let bloom 5 minutes. Put white chocolate and 1/4 cup milk in a heatproof bowl. Heat milk until steaming (microwave 30–45 sec or stovetop), pour over chocolate and let sit 1 minute, then whisk until smooth. If needed, microwave in 10-second bursts to finish melting.

Incorporate gelatin

Warm bloomed gelatin 5–8 seconds in microwave until liquid, then whisk into melted white chocolate until fully incorporated and glossy. Whisk in vanilla.

Whip cream

Whip 1.5 cups cold heavy cream to soft-medium peaks (starts to hold shape but still glossy). Reserve a few tablespoons for smoothing if desired.

Fold together

Fold one-third of whipped cream into the white chocolate to loosen the mixture, then gently fold in remaining cream until homogeneous and airy. Chill briefly (15–20 minutes) if mixture feels too soft to layer.

Bloom gelatin & melt chocolate

Sprinkle 1 tsp gelatin over 2 tbsp cold water; let bloom 5 minutes. Place milk chocolate and 1/4 cup milk in a heatproof bowl. Heat milk until steaming, pour over chocolate, let sit 1 minute, then whisk until smooth.

Add gelatin & season

Warm bloomed gelatin 5–8 seconds until liquid and whisk into melted milk chocolate. Stir in 1/8 tsp salt to deepen flavor.

Whip cream

Whip 1.5 cups cold heavy cream to soft-medium peaks.

Fold together

Fold a third of the whipped cream into the melted milk chocolate to lighten, then fold in the rest gently until even. Chill 10–20 minutes if too soft for assembly.

Heat cream

Place 3/4 cup heavy cream in a small saucepan and bring just to a simmer over medium heat (do not boil). Remove from heat.

Make ganache

Pour hot cream over chopped dark chocolate in a heatproof bowl. Let sit 1 minute, then stir until smooth. Add butter and corn syrup, stir until glossy. Let cool at room temperature until thick but pourable (about 20–30 minutes). If too thin, chill briefly and stir; if too firm, warm gently.

Layering

Place one cake layer on a cake board or serving plate. If cake is domed, place cut side up so layers sit flat. Spread a thick, even layer (about half) of milk chocolate mousse over the cake, smoothing to edges. Chill 15 minutes to set slightly. Place second cake layer on top and press gently. Spread white chocolate mousse over the top and smooth edges or pipe a neat edge if desired. Chill 1–2 hours (or until mousse is set) in refrigerator.

Apply truffle icing

When mousse is set and ganache is pourable but not hot, pour ganache over center of chilled cake and use an offset spatula to gently coax it over edges for a smooth, even coat. Work quickly; the cooled mousse will help ganache set. If you prefer a drip effect, allow more ganache to run over edges.

Finish & garnish

Chill cake 30 minutes to let ganache set. Decorate with chocolate shavings or cocoa nibs around the top edge if desired. Let cake sit at cool room temperature 10–15 minutes before slicing for cleaner cuts. Store refrigerated up to 3 days.