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

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

In a large mixing bowl, beat the softened butter and sugar until light and fluffy, about 3-5 minutes.

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

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

Pour the batter into the prepared cake pan and smooth the top. Bake for 25-30 minutes or until a toothpick comes out clean.

Allow the cake to cool in the pan for 10 minutes, then transfer to a wire rack to cool completely.

For the raspberry mousse, puree raspberries with sugar in a blender, then strain through a fine mesh sieve.

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

Refrigerate the mousse for about 30 minutes to set.

Once the cake is cooled, slice it in half horizontally to create two layers.

Place one layer on a serving plate and spread half of the raspberry mousse over it, then add the second layer and top with remaining mousse.

Chill the assembled cake in the refrigerator for at least 1 hour.

For the ganache, heat heavy cream until just boiling, then pour over chopped dark chocolate and let sit for a few minutes before stirring until smooth.

Drizzle the ganache over the chilled cake before serving.