Preheat the oven to 350°F (175°C). Grease and flour two 9-inch round cake pans. In a large bowl, mix together flour, cocoa powder, sugar, baking powder, baking soda, and salt. Add eggs, milk, oil, and vanilla. Mix until smooth. Stir in boiling water until well combined. Pour batter into prepared pans.

Bake the Cake

Bake in the preheated oven for 30-35 minutes, or until a toothpick inserted into the center comes out clean. Let cool in the pans for 10 minutes, then remove from pans and transfer to wire racks to cool completely.

Fudge Preparation

Prepare the Chocolate Fudge

In a saucepan, heat heavy cream and butter until just boiling. Remove from heat and add chopped dark chocolate. Stir until smooth and glossy. Let it cool slightly before using.

Mousse Preparation

Make Milk Chocolate Mousse

Melt the milk chocolate over a double boiler. In a small bowl, bloom gelatin in water. In a separate bowl, whip heavy cream to soft peaks. Fold melted chocolate and gelatin into the whipped cream, then add vanilla. Chill until set.

Make Dark Chocolate Mousse

Melt the dark chocolate over a double boiler. In a bowl, whip egg whites until soft peaks form, gradually adding sugar until glossy. Fold chocolate into whipped egg whites. Chill until set.

Crèmeaux Preparation

Prepare White Chocolate Crèmeaux

In a saucepan, heat milk and sugar over medium heat until just boiling. In a bowl, whisk egg yolks, then slowly add the hot milk mixture while whisking continuously. Return to the saucepan and cook until thickened. Remove from heat, add chopped white chocolate, and stir until smooth.

Assembly

Assemble the Cake

Layer the cooled chocolate cake with chocolate fudge, milk chocolate mousse, dark chocolate mousse, and top with white chocolate crèmeaux. Repeat layers as desired. Chill before serving.