Make the Chocolate Cake:
Preheat the oven to 350°F (180°C). Grease two 8-inch cake pans.
Sift together flour, sugar, cocoa powder, baking powder, baking soda, and salt in a large bowl.
In another bowl, mix eggs, milk, sour cream, vegetable oil, and vanilla until combined.
Gradually mix dry ingredients into wet ingredients. Dissolve espresso powder in hot water and add to the batter. Mix until smooth.
Divide batter evenly between prepared pans and bake for 35 minutes or until a skewer comes out clean. Allow to cool completely.
Make the Chocolate Mousses:
6. Chop dark and white chocolates, placing each in separate bowls. Whisk egg yolks until pale. Heat half the cream with vanilla until warm, then gradually whisk into yolks. Return mixture to heat and cook until it thickens.
7. Divide cream mixture between chocolates; stir until melted. Whip remaining cream to stiff peaks and fold into chocolate mixtures separately.
8. Place one cake layer in a cake ring. Pour dark chocolate mousse over, then white chocolate mousse. Refrigerate for 30 minutes, add second cake layer, and chill for 4 hours.
Make the Chocolate Ganache:
9. Heat cream until just scalding, pour over milk chocolate, and stir until smooth. Let cool to a spreadable consistency, then spread over the top of the cake.
To Serve:
Remove cake from the cake ring, spread ganache over the top, and decorate as desired.