Preheat your oven to 350°F (175°C) and prepare an 8-inch round cake pan by greasing it with butter and lining with parchment paper.

Whisk together flour, cocoa powder, baking powder, baking soda, and salt in a large mixing bowl.

Beat softened butter and sugar together until light and fluffy.

Add eggs one at a time, mixing well after each addition, then stir in vanilla extract.

Gradually add dry ingredients to the butter mixture, alternately with buttermilk, mixing just until combined.

Pour the batter into the prepared pan and bake for 25-30 minutes, or until a toothpick comes out clean.

Let the cake cool in the pan for 10 minutes before transferring to a wire rack to cool completely.

Puree fresh raspberries with sugar in a blender and strain through a fine mesh sieve.

Whip heavy cream until soft peaks form, then fold in melted dark chocolate and strained raspberry puree.

Slice the cooled cake horizontally, spread raspberry mousse on the first layer, then place the second layer on top and spread remaining mousse.

Chill in the refrigerator for at least 1 hour.

Heat heavy cream until boiling, pour over chopped dark chocolate, and stir until smooth to make ganache.

Drizzle ganache over the chilled cake and garnish with fresh raspberries and mint before serving.