Preheat your oven to 350°F (175°C) and prepare two 9-inch round cake pans by greasing and lining with parchment paper.

In a large mixing bowl, combine the all-purpose flour, granulated sugar, cocoa powder, baking powder, baking soda, and salt. Whisk until evenly blended.

In another bowl, whisk together the eggs, whole milk, sour cream, vegetable oil, espresso powder, and vanilla extract until smooth.

Gently fold the wet ingredients into the dry mixture and gradually pour in the hot water, stirring until the batter is smooth.

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

Cool the cakes in the pans for 10 minutes before transferring to wire racks to cool completely for about 2 hours.

In a saucepan, heat 1 cup of heavy cream until simmering. In another bowl, whisk the egg yolks, sugar, and cornstarch until combined.

Whisk the simmering cream into the egg mixture to create a custard base for the mousses, then divide into two bowls.

In one bowl, add melted dark chocolate and in the other, fold in melted white chocolate until smooth.

Gently fold whipped heavy cream into each chocolate mixture.

Place one cooled cake layer on a serving plate and spread with dark chocolate mousse, add the second layer, and cover with white chocolate mousse.

Wrap the assembled cake with plastic wrap and refrigerate for at least 2 hours to firm up the mousses.

Heat 1 cup of heavy cream for ganache until hot, pour over chopped milk chocolate and stir until smooth. Allow to cool slightly.

Pour ganache over the chilled cake and refrigerate for another 30 minutes before serving.