Preheat your oven to 350°F (175°C) and prepare your 8-inch round cake pan by greasing it with unsalted butter or non-stick spray and dusting with flour.

In a large mixing bowl, whisk together the all-purpose flour, unsweetened cocoa powder, baking powder, baking soda, and salt until well combined.

Beat the softened unsalted butter and granulated sugar with an electric mixer on medium speed for about 3-5 minutes until light and fluffy. Then add the eggs one at a time, mixing well after each, and stir in vanilla.

Gradually add the dry mixture to the creamed butter and sugar, alternating with the buttermilk. Mix on low speed until just combined.

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

Let the cake cool in the pan for about 10 minutes, then invert onto a wire rack to cool completely.

Puree the fresh raspberries with sugar in a blender until smooth, then strain the puree through a fine sieve to remove seeds.

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

Fold the melted dark chocolate into the whipped cream until combined, then gently fold in the raspberry puree.

Once the cake is cooled, slice it horizontally into two layers. Spread raspberry mousse over the first layer, then place the second layer on top and spread remaining mousse.

Cover the cake and chill in the refrigerator for at least 1 hour.

To make the ganache, heat the heavy cream until boiling, then pour over the chopped dark chocolate and stir until smooth.

Drizzle the ganache over the chilled cake, allowing it to cascade down the sides.

Slice and serve with fresh raspberries or whipped cream.