Preheat oven to 350°F (175°C) and prepare an 8-inch round cake pan by greasing and dusting with flour.

In a medium bowl, whisk together 1 ¾ cups all-purpose flour, ¾ cup unsweetened cocoa powder, 1 ½ teaspoons baking powder, ½ teaspoon baking soda, and ½ teaspoon salt.

In a large bowl, cream together ½ cup softened unsalted butter and 1 ½ cups granulated sugar until light and fluffy.

Add 3 large eggs one at a time, mixing well after each addition, then stir in 1 teaspoon of pure vanilla extract.

Gradually add dry mixture to the creamed mixture, alternating with 1 cup of buttermilk, mixing just until combined.

Pour batter into the prepared pan and bake for 25-30 minutes, or until a toothpick comes out clean. Cool in the pan for 10 minutes, then transfer to a wire rack.

Blend 1 cup fresh raspberries with 2 tablespoons granulated sugar until smooth, then strain.

Whip 1 cup heavy cream and 1 teaspoon vanilla extract to soft peaks, fold in 4 ounces melted dark chocolate and raspberry puree. Chill for at least 30 minutes.

Slice the cooled cake in half horizontally. Spread half of the mousse on the bottom layer, then place the top layer and smooth remaining mousse on top.

For ganache, heat ½ cup heavy cream to a gentle boil, pour over 6 ounces chopped dark chocolate and whisk until smooth.

Drizzle ganache over the chilled mousse cake and garnish with fresh raspberries and cocoa powder.