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

In a medium bowl, whisk together the flour, cocoa powder, baking powder, baking soda, and salt until well combined. Set aside.

In a large mixing bowl, beat the butter and sugar until light and fluffy, then add eggs one at a time, mixing well after each addition along with vanilla extract.

Gradually mix in the dry ingredients and buttermilk alternately until combined. Avoid overmixing.

Pour the batter into the prepared pan and bake for 25-30 minutes or until a toothpick comes out clean. Cool for 10 minutes.

Remove the cake from the pan and let it cool completely on a wire rack.

Prepare the mousse by pureeing the fresh raspberries with sugar, then strain. Whip heavy cream and fold in chocolate and raspberry puree.

Slice the cooled cake in half horizontally. Place one layer on a plate, spread with half of the mousse, add the second layer, and top with remaining mousse.

Chill the assembled cake for at least 1 hour covered with plastic wrap.

For the ganache, heat heavy cream to boiling and pour over chopped chocolate, stirring until smooth. Drizzle over the chilled cake before serving.