Dark/white Chocolate Mousse Cake Recipe

Prepare

Preheat oven to 350°F (175°C). Lightly butter a 9-inch springform pan and line the bottom with parchment paper.

Whisk together flour, cocoa powder, sugar, baking powder, baking soda, and salt in a medium bowl until evenly combined.

In a separate bowl, whisk eggs, milk, oil, and vanilla until combined.

Pour wet ingredients into dry ingredients and stir just until combined. Gradually add the hot water and whisk until batter is smooth and slightly thin — this keeps the sponge moist.

Pour batter into prepared pan and bake 18–22 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs. Avoid overbaking.

Cool in pan for 10 minutes, then remove ring and transfer cake with parchment to a wire rack to cool completely. For assembly, you want the cake fully cooled; you can chill 30 minutes to firm it.

Place chopped dark chocolate in a medium heatproof bowl.

In a small saucepan, bring 1/2 cup of the heavy cream to a simmer. Pour over the chopped chocolate and let sit 1 minute, then stir gently until smooth and glossy. Stir in softened butter and salt. Set ganache aside to cool until lukewarm.

While ganache cools, whisk the 3 cold egg yolks with 1 tablespoon sugar in a heatproof bowl until slightly pale. Heat the remaining 1 cup heavy cream to just under a simmer (do not boil). Temper the hot cream into the yolks: add a few tablespoons of hot cream to the yolks while whisking, then slowly whisk the rest in.

Return the yolk-cream mixture to the saucepan and cook over low heat, stirring constantly, until it thickens slightly and coats the back of a spoon (around 170°F/77°C). Remove from heat and cool slightly.

Stir the warm yolk custard into the dark chocolate ganache until smooth. If using gelatin for extra stability: bloom 1 tsp gelatin in 2 tbsp cold water for 5 minutes, then warm gently until melted and stir into the chocolate mixture.

Allow the mixture to cool to room temperature, stirring occasionally. Meanwhile whip the remaining (if any) heavy cream — use remaining 0.5 cup if needed — to soft peaks. Fold whipped cream into the chocolate mixture gently in three additions to create a light mousse. Chill briefly (15–20 minutes) to firm if needed before using.

Place chopped white chocolate in a medium heatproof bowl.

In a small saucepan, bring 1/2 cup of the heavy cream to a simmer. Pour over the chopped white chocolate and let sit 1 minute, then stir until smooth. Stir in softened butter and vanilla. Set aside to cool until lukewarm.

Whisk the 3 cold egg yolks with 1 tablespoon sugar until slightly pale. Heat the remaining 1 cup heavy cream to just under a simmer. Temper the hot cream into the yolks by adding a few tablespoons first, then the rest while whisking.

Return mixture to the saucepan and cook over low heat, stirring constantly, until it thickens slightly and coats the back of a spoon (~170°F/77°C). Remove from heat and let cool briefly.

Stir the warm yolk custard into the white chocolate mixture until smooth. If using gelatin for extra stability, bloom and add as directed for the dark mousse.

Cool to room temperature. Whip remaining heavy cream to soft peaks and fold into the white chocolate base in three additions until light and homogenous. Chill briefly if needed before assembling.

Level the cooled sponge if needed. Place the sponge back in the 9-inch springform ring on a parchment-lined plate or cake board to form a stable base.

Spread the dark chocolate mousse evenly over the sponge to a thickness of about 3/4 inch (or to about half the final mousse height). Smooth the top with an offset spatula. Chill in the refrigerator 20–30 minutes to set slightly.

Once the dark layer is lightly set, spoon the white chocolate mousse on top and smooth to create the top layer. Tap the cake gently on the counter to remove air pockets and level the surface.

Refrigerate the assembled cake at least 4 hours, preferably overnight, to fully set the mousses. If you used gelatin, this time can be shorter (2–3 hours).

Before serving, run a thin knife around the springform ring and remove it. Dust the top lightly with cocoa powder and decorate with chocolate shavings as desired. Slice with a hot dry knife (wipe between cuts) for clean slices.