Preheat the oven to 350 degrees F. Grease a 9-inch springform pan and line the bottom with parchment paper.

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

Add the buttermilk, vegetable oil, eggs, and vanilla extract. Mix until the batter is smooth.

Carefully pour in the boiling water. The batter will be thin, but that is normal. Stir just until combined.

Pour the batter into the prepared pan. Bake for 22 to 25 minutes, or until a toothpick inserted in the center comes out clean.

Let the cake cool in the pan for 10 minutes. Then remove it from the pan and let it cool completely on a wire rack.

Clean the springform pan and line the sides with parchment paper or an acetate collar. Place the cooled cake layer back inside the pan to create a mold for the mousse layers.

Begin the dark chocolate mousse layer. Heat 1/2 cup of heavy cream until steaming, but do not let it boil. Pour the hot cream over the chopped dark chocolate. Let it sit for one minute, then stir until smooth. Let the mixture cool to room temperature.

Whip the remaining 1 cup of heavy cream with 3 tablespoons of sugar until soft peaks form. Fold the whipped cream into the cooled dark chocolate mixture in two additions, being gentle to keep the mousse airy.

Spread the dark chocolate mousse over the cake layer. Tap the pan lightly on the counter to release air bubbles. Refrigerate for 20 to 30 minutes while you prepare the next layer.

Repeat the same process for the milk chocolate mousse. Heat 1/2 cup of heavy cream, pour it over the chopped milk chocolate, stir until smooth, and cool. Whip the remaining cream with sugar, fold, and spread over the chilled dark chocolate layer. Refrigerate again for 20 to 30 minutes.

Prepare the white chocolate mousse the same way. White chocolate can be more heat-sensitive, so warm the cream gently and stir slowly to prevent separation.

Spread the white chocolate mousse evenly over the top. Cover the pan loosely with plastic wrap and refrigerate for at least 5 hours, preferably overnight.

To serve, run a thin knife around the edge of the pan, release the springform, and remove the parchment collar. Garnish with chocolate shavings or curls.