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

In a medium bowl, whisk together the all-purpose flour, unsweetened cocoa powder, baking powder, baking soda, and salt until evenly combined.

In a large mixing bowl, beat the softened unsalted butter and granulated sugar together using a mixer for about 3-5 minutes until light and fluffy.

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

Gradually incorporate the dry ingredients into the wet mixture while alternately adding 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. Let cool in the pan for 10 minutes before transferring to a wire rack.

Puree fresh raspberries with granulated sugar in a blender until smooth. Strain through a fine mesh sieve and set aside. Whip the heavy cream with vanilla until soft peaks form.

Fold the melted dark chocolate into the whipped cream until well combined and gently add the raspberry puree. Chill the mousse for at least 30 minutes.

Once the cake has cooled, slice it in half horizontally. Spread half of the raspberry mousse on the bottom layer, then replace the top layer and spread the remaining mousse on top.

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

Heat the heavy cream for the ganache until it begins to boil. Pour over chopped dark chocolate and let sit before stirring for a smooth consistency.

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