Preheat your oven to 350°F (175°C). Grease and line two 9-inch round cake pans with parchment paper. Whisk together the dry ingredients until well combined.

Combine the wet ingredients in a separate bowl and whisk until smooth.

Gradually add the wet mixture to the dry ingredients, stirring gently. Slowly pour in the hot water and mix until smooth.

Divide the batter between the prepared pans and bake for 30-35 minutes, until a toothpick comes out clean.

While cakes cool, heat heavy cream until it simmers. Whisk together egg yolks, sugar, and cornstarch. Carefully whisk in hot cream, then cook until thickened. Mix dark chocolate into one half and white chocolate into the other until melted.

Whip the remaining heavy cream to soft peaks and fold into the chocolate mixtures.

Place one cake layer on a plate, spread dark chocolate mousse on top, and add the second layer. Top with white chocolate mousse and refrigerate for 2 hours.

Heat remaining cream until it simmers, pour over chopped milk chocolate, and stir until smooth to prepare ganache.

Pour ganache over the cake and refrigerate for 30 minutes to set.