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

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

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

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

Gradually mix in the dry ingredients and buttermilk until just combined.

Pour the batter into the prepared pan and smooth the top.

Bake for 25-30 minutes, or until a toothpick inserted comes out clean. Cool in the pan for 10 minutes before transferring to a wire rack to cool completely.

Puree the raspberries with granulated sugar in a blender, then strain through a fine mesh sieve.

Whip heavy cream and vanilla extract until soft peaks form. Fold in melted dark chocolate and strained raspberry puree.

Chill the mousse in the refrigerator for about 30 minutes to set.

Once the cake is completely cooled, slice it into two equal layers.

Position one layer on a serving plate and spread half of the raspberry mousse on top.

Place the second layer on top and spread the remaining mousse over the entire cake.

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

Heat heavy cream until just boiling, then pour it over chopped dark chocolate in a bowl.

Let it sit for a few minutes to soften, then stir until smooth. Drizzle over the chilled cake just before serving.