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

In a medium bowl, whisk together flour, cocoa powder, baking powder, baking soda, and salt.

In a large bowl, beat the butter and sugar until light and fluffy. Add eggs, one at a time, and mix well. Stir in vanilla extract.

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

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

Puree raspberries with sugar, then strain the mixture for a smooth puree. Whip cream until soft peaks form.

Fold in melted dark chocolate and raspberry puree. Chill for at least 30 minutes.

Slice the cooled cake horizontally and layer with raspberry mousse. Chill for 1 hour.

Heat heavy cream until it boils, then pour over chopped chocolate for ganache. Stir until smooth.

Drizzle ganache over the chilled cake before serving.