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

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

In a separate bowl, mix together the whole milk, large eggs, sour cream, vegetable oil, espresso powder, and vanilla extract. Whisk until smooth.

Gently pour the wet mixture into the dry ingredients and stir until just combined. Slowly add the hot water to the batter.

Divide the batter equally between the two prepared cake pans. Bake for 30-35 minutes, or until a toothpick comes out clean.

In a saucepan, heat the heavy cream over medium heat until simmering. Whisk together egg yolks, sugar, and cornstarch in a bowl.

Gradually pour the cream into the egg mixture while whisking. Return to saucepan and cook until thickened, then remove from heat.

Divide the custard; stir melted dark chocolate into one for dark chocolate mousse and white chocolate into the other for white chocolate mousse.

Place one chocolate cake layer on a serving plate, spread dark chocolate mousse over it, and top with the second cake layer.

Spread white chocolate mousse over the top layer and refrigerate for at least 2 hours.

For the ganache, heat the heavy cream and pour over the chopped milk chocolate, stirring until melted and smooth.

Pour the ganache over the chilled mousse cake and refrigerate for an additional 30 minutes to set.