Preheat your oven to 350°F (175°C). Grease and flour an 8-inch round cake pan, ensuring an even layer of butter and flour.

In a large mixing bowl, whisk together all-purpose flour, unsweetened cocoa powder, baking powder, baking soda, and salt until no lumps remain.

In another bowl, beat the softened unsalted butter and granulated sugar together until light and fluffy. Add eggs one at a time, mixing well after each addition, then stir in vanilla extract.

Gradually add the dry ingredients into the butter mixture, alternating with buttermilk. Mix until just combined, then pour the batter into the prepared pan.

Bake for 25-30 minutes or until a toothpick inserted in the center comes out clean. Allow the cake to cool for 10 minutes before transferring to a wire rack.

While the cake cools, prepare the raspberry mousse by pureeing fresh raspberries with sugar, straining, and whipping heavy cream with vanilla until soft peaks form. Fold in melted dark chocolate and raspberry puree.

Slice the cooled cake horizontally into two even layers. Spread half of the raspberry mousse on the bottom layer, place the top layer, and spread the remaining mousse on top.

Refrigerate the assembled cake for at least 1 hour to set. As it chills, admire your creation.

For the ganache, heat heavy cream until simmering, pour over chopped dark chocolate, let sit, then stir until smooth.

Drizzle ganache over the chilled cake before serving. Optionally decorate with fresh raspberries or whipped cream.