1. Baking the Chocolate Cake

Begin by preheating your oven to 350°F (175°C). Grease a 9-inch cake pan with baking spray and line the bottom with parchment paper for easy removal after baking. Set aside.

In a large bowl, whisk together the flour, sugar, cocoa powder, baking soda, salt, and baking powder until evenly combined. Create a well in the center of the dry ingredients, and into this well, add the eggs, milk, melted butter, and vanilla extract. Stir until everything is just combined, scraping the sides of the bowl as needed.

Add the boiling water to the batter and whisk again until smooth. The batter will be quite thin, but that’s what creates the light and fluffy texture once baked. Pour the batter into your prepared pan and bake for 28-30 minutes or until a toothpick inserted into the center comes out clean with just a few moist crumbs. Let the cake cool in the pan for 5 minutes, then transfer it to a wire rack to cool completely.

Once the cake is cool, if necessary, trim the top of the cake to level it. Place the cake onto your cake plate and wrap it securely with a layer of plastic acetate. Alternatively, you can place it back in the springform pan. This will help keep the mousse layers in place and create a neat finish.

2. Making the Dark Chocolate Mousse

Start by blooming the gelatin. In a small bowl, sprinkle the gelatin over the cold water and let it sit for 8-10 minutes to absorb the liquid and expand.

In a separate heatproof bowl, place the dark chocolate and melt it gently over a double boiler, stirring frequently to prevent burning. Once melted and smooth, set aside to cool slightly.

After the gelatin has bloomed, microwave it for 10 seconds to melt it, and then whisk it into the melted dark chocolate. Set this aside to cool while you whip the cream.

In a mixing bowl, beat the heavy cream with the granulated sugar using a stand or hand mixer until soft peaks form—this should take around 2-4 minutes on medium speed.

Carefully fold the whipped cream into the dark chocolate mixture using a rubber spatula until fully incorporated, being careful not to deflate the cream. Spoon the mousse over the cooled cake layer in your cake pan, smoothing it out evenly. Refrigerate for at least 2 hours to set.

3. Making the White Chocolate Mousse

Repeat the same blooming process for the gelatin by dissolving it in cold water and letting it sit for 8-10 minutes.

Meanwhile, place the white chocolate in another heatproof bowl. In a saucepan, bring 1 cup of heavy cream to a simmer. Pour the hot cream over the white chocolate, then whisk until the chocolate is completely melted and smooth. Add the bloomed gelatin and whisk until fully dissolved. Allow this mixture to cool to room temperature.

While the white chocolate mixture is cooling, whip the remaining 1 cup of heavy cream with sugar to soft peaks. Once the white chocolate mixture has cooled, gently fold it into the whipped cream until fully combined.

Pour the white chocolate mousse over the dark chocolate mousse layer and smooth the top. Refrigerate for another 3 hours or overnight to ensure the mousse layers are completely set.

4. Making the Chocolate Ganache

To make the luscious chocolate ganache, place the semi-sweet chocolate in a bowl. In a small saucepan, bring 1 cup of heavy cream to a bare simmer. Pour the hot cream over the chocolate and let it sit for 1 minute to soften. Stir until the chocolate is completely melted and smooth.

Allow the ganache to cool slightly for about 10 minutes until it thickens but remains pourable. Carefully pour the ganache over the set white chocolate mousse, letting it drip down the sides of the cake for that irresistible effect.

5. Garnishing the Cake

Once the ganache has set, carefully remove the acetate film. For a festive touch, garnish the top of your cake with fresh rosemary, sage, and sugared cranberries. For a more casual or non-holiday look, fresh berries such as raspberries or blueberries make a beautiful and equally delicious garnish.