1. Preheat your oven to 350°F and grease an 8-inch round cake pan. In a mixing bowl, whisk together the flour, cocoa powder, baking powder, baking soda, and salt. In a separate bowl, cream the softened butter and sugar until light and fluffy. Beat in the eggs one at a time, then stir in the vanilla. Alternate adding the dry ingredients and buttermilk to the butter mixture, mixing until the batter is smooth. Pour the batter into the prepared pan and bake for 25-30 minutes, or until a toothpick inserted into the center comes out clean. Allow the cake to cool completely on a wire rack.

2. In a blender, puree the fresh raspberries with the granulated sugar until smooth. Strain the mixture through a fine-mesh sieve to remove the seeds. In a separate bowl, whip the heavy cream with the vanilla until stiff peaks form. Gently fold in the melted dark chocolate and the raspberry puree until the mousse is well combined and silky smooth. Refrigerate the mousse until it’s slightly set, about 30 minutes.

3. If desired, slice the cooled chocolate cake horizontally to create layers. Spread the raspberry mousse between the cake layers and over the top of the cake, smoothing it with a spatula. Refrigerate the assembled cake for at least 2 hours to allow the mousse to set.

4. In a small saucepan, heat the heavy cream just until it begins to boil. Remove from heat and pour over the chopped dark chocolate. Let sit for 1-2 minutes, then stir until smooth. Pour the ganache over the chilled cake, allowing it to drip down the sides.

5. Refrigerate the assembled cake for at least 2 hours, or until the ganache has set. Garnish with fresh raspberries before serving.