Preheat your oven to 350°F (175°C). Grease and flour an 8-inch round cake pan.

In a medium bowl, whisk together 1 cup of all-purpose flour, 1/2 cup of unsweetened cocoa powder, 1 teaspoon of baking powder, 1/2 teaspoon of baking soda, and 1/4 teaspoon of salt.

In a separate large bowl, beat 1/2 cup of softened unsalted butter and 1 cup of granulated sugar together using an electric mixer for about 3-5 minutes until light and fluffy.

Add 2 large eggs, one at a time, mixing well after each addition. Stir in 1 teaspoon of vanilla extract until fully combined.

Gradually add the dry mixture to the wet mixture, alternating with 1/2 cup of buttermilk until just combined.

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

For the raspberry mousse, blend 1 cup of fresh raspberries with 1/4 cup of sugar until smooth, then strain. In another bowl, whip 1 cup of heavy cream with 1 teaspoon of vanilla extract until soft peaks form. Fold in melted dark chocolate and raspberry puree, then refrigerate for 30 minutes.

Once cooled, slice the cake in half horizontally. Spread half of the raspberry mousse over the bottom layer, place the second layer on top, and spread remaining mousse on top. Chill for 1 hour.

To make the ganache, heat 1/2 cup of heavy cream until boiling. Pour over 4 ounces of chopped dark chocolate, let sit for a few minutes, then stir until smooth.

Drizzle the ganache over the chilled cake and serve with fresh raspberries or whipped cream.