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

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

In a large mixing bowl, beat the softened unsalted butter and granulated sugar together with an electric mixer until light and fluffy.

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

Alternate adding the dry mixture and buttermilk into the creamed mixture, starting and ending with the dry ingredients.

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

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

While the cake cools, puree fresh raspberries with sugar until smooth, and strain through a fine sieve.

In a large bowl, whip the heavy cream with vanilla extract until soft peaks form.

Fold the melted dark chocolate into the whipped cream mixture, followed by the strained raspberry puree.

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

Once the cake is cool, slice it horizontally into two layers. Spread half of the mousse on the bottom layer and place the second layer on top.

Spread the remaining mousse on top and refrigerate for at least 1 hour.

To prepare the ganache, heat heavy cream until just simmering and pour over the chopped dark chocolate, stirring until smooth.

Drizzle the ganache over the chilled cake and allow it to set slightly before slicing.