Preparation:

Step 1: Bake the Cake Base

Preheat the oven to 350°F (175°C). Prepare your chocolate cake mix according to the instructions on the box or your preferred recipe. Combine the cake mix with water, vegetable oil, and egg, then mix until smooth. Pour the batter into a greased 9-inch springform pan and bake for 20-25 minutes or until a toothpick inserted into the center comes out clean. Allow the cake to cool completely in the pan.

Step 2: Make the Dark Chocolate Mousse

In a saucepan, heat 1/2 cup of heavy cream over medium heat until it just begins to simmer. Pour the hot cream over 1 cup of dark chocolate chips in a heatproof bowl. Let it sit for about 2 minutes to melt the chocolate, then stir until smooth. Let the mixture cool slightly. Whip the remaining 1/2 cup of heavy cream until soft peaks form, then gently fold it into the chocolate mixture until fully incorporated. Spread the dark chocolate mousse evenly over the cooled cake base. Chill the cake in the refrigerator for at least 1 hour, or until the mousse is set.

Step 3: Make the Milk Chocolate Mousse

Repeat the process for the milk chocolate mousse. Heat 1/2 cup of heavy cream in a saucepan until it begins to simmer, then pour it over 1 cup of milk chocolate chips. Stir until smooth, then let it cool slightly. Whip the remaining 1/2 cup of heavy cream and fold it into the milk chocolate mixture. Once the dark chocolate mousse layer has set, spread the milk chocolate mousse over it. Chill again until set.

Step 4: Make the White Chocolate Mousse

For the white chocolate mousse, repeat the same steps: heat 1/2 cup of heavy cream and pour it over the white chocolate chips. Stir until smooth and let it cool slightly. Whip the remaining 1/2 cup of heavy cream and fold it into the white chocolate mixture. Spread this final mousse layer over the chilled milk chocolate mousse. Refrigerate the cake for at least 2 hours, or until the mousse layers are completely set.

Step 5: Serve

Once the mousse layers are fully set, carefully release the springform pan. Slice the cake into portions and serve. For a finishing touch, garnish with chocolate shavings, cocoa powder, or fresh berries.