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

In a large bowl, sift together the dry ingredients: flour, sugar, cocoa powder, baking powder, baking soda, and salt.

In a separate bowl, whisk together the wet ingredients: milk, eggs, sour cream, vegetable oil, espresso powder, and vanilla until smooth.

Gradually combine the wet and dry mixtures, stirring gently before adding hot water to create a glossy batter.

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

To prepare the mousse, heat heavy cream in a saucepan until simmering. In another bowl, whisk egg yolks, sugar, and cornstarch until smooth.

Slowly pour the hot cream into the egg mixture while whisking to temper the eggs, then cook on low until thickened.

Divide the mixture into two bowls and stir melted dark chocolate into one, and melted white chocolate into the other.

Fold whipped cream into each bowl of mousse gently until just combined to maintain airiness.

Layer the cake by placing one cake layer on a serving plate, spreading dark mousse on top, then adding the second layer and white mousse on top.

Cover and refrigerate for at least 2 hours to set the mousse layers. Prepare the ganache during this time.

For the ganache, heat heavy cream until simmering, pour it over chopped milk chocolate, and stir until smooth.

Once the cake is chilled, pour the ganache over the top and let it set in the refrigerator for an additional 30 minutes before slicing.