Preheat your oven to 350°F (175°C) while you grease and line two 9-inch round cake pans with parchment paper. Set aside.

In a large mixing bowl, whisk together the all-purpose flour, granulated sugar, cocoa powder, baking powder, baking soda, and salt until smooth.

In another bowl, combine the wet ingredients: whole milk, large eggs, sour cream, and vegetable oil. Mix well and add vanilla extract and espresso powder if using.

Slowly add the wet mixture into the dry ingredients, stirring gently. Gradually add hot water to achieve a smooth batter.

Pour the batter evenly into prepared pans and bake for 30–35 minutes until a toothpick comes out clean. Cool in pans for 10 minutes.

Prepare the dark chocolate mousse by heating half the heavy cream until simmering, then adding it to the egg mixture and melted dark chocolate.

Repeat the process for the white chocolate mousse with the remaining heavy cream.

On a serving plate, place one cooled cake layer and spread the dark chocolate mousse over it. Top with the second layer and white chocolate mousse.

Refrigerate the assembled cake for at least 2 hours to set the mousses.

Make the ganache by heating the remaining heavy cream and pouring it over the milk chocolate. Stir until smooth.

Pour the ganache over the chilled cake and smooth it out. Return to the refrigerator for 30 minutes before serving.