Preheat oven to 350°F (175°C). Grease and flour two 9-inch round cake pans. Whisk together flour, sugar, cocoa powder, baking powder, baking soda, and salt in a large bowl.

In another bowl, mix buttermilk, vegetable oil, eggs, and vanilla extract until well combined.

Gradually add wet ingredients to dry ingredients, mixing on low speed until just combined. Slowly pour in hot coffee while mixing.

Divide batter evenly between cake pans. Bake for 30-35 minutes, or until a toothpick inserted into the center comes out clean. Cool in pans for 10 minutes before inverting onto a wire rack to cool completely.

For the mousse, heat heavy cream over medium heat until just about to simmer. Remove from heat and add chopped milk chocolate. Let sit for a minute, then stir until smooth and glossy. Stir in vanilla extract.

Pour mousse mixture into a bowl, cover with plastic wrap (pressing onto surface), and refrigerate for at least 2 hours, or until thickened but spreadable.

Level cake layers if uneven. Place one layer on a serving plate, spread half of the mousse over top. Place the second layer on top, and frost the top and sides with remaining mousse.

Refrigerate assembled cake for at least another hour to set the mousse.