Preheat the 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 mixing bowl, whisk together the dry ingredients: flour, sugar, cocoa powder, baking powder, baking soda, and salt until well blended.

In a separate bowl, combine the wet ingredients: milk, eggs, sour cream, vegetable oil, espresso powder, and vanilla extract, then mix them well.

Gradually add the wet mixture to the dry ingredients and mix gently until just combined. Stir in the hot water until the batter is smooth.

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

Let the cakes cool in the pans for 10 minutes, loosen the edges, and invert them onto wire racks to cool completely for at least 2 hours.

For the mousses, heat 1 cup of heavy cream until simmering. Whisk egg yolks, sugar, and cornstarch in a bowl, and gradually add the hot cream, whisking continuously.

Return the mixture to heat and cook until thickened, about 5 minutes. Divide and stir dark chocolate into one bowl and white chocolate into another.

Whip the remaining heavy cream to soft peaks, then fold into the chocolate mixtures to create fluffy mousses.

Assemble the cake by placing one layer on a serving plate. Spread dark chocolate mousse on top, add the second layer, and spread white chocolate mousse. Refrigerate for at least 2 hours.

For the ganache, heat heavy cream until steaming, pour it over chopped milk chocolate, let it sit for 5 minutes, and stir until smooth.

Pour the ganache over the cooled assembled cake and let it set in the refrigerator for 30 minutes before slicing.