Preheat your oven to 350°F (175°C) and grease an 8-inch round cake pan with butter, then dust it with flour. In a large mixing bowl, whisk together all-purpose flour, unsweetened cocoa powder, baking powder, baking soda, and salt until well combined.

In a stand mixer or large bowl, beat the softened unsalted butter and granulated sugar together for 3-5 minutes until light and fluffy. Add in the eggs one at a time, mixing well after each addition, then stir in the vanilla extract. Gradually combine the dry mixture with buttermilk, mixing until the batter is just smooth and ready to pour into the prepared pan.

Pour the rich batter into the greased cake pan, spreading it evenly. Bake in the preheated oven for 25-30 minutes, or until a toothpick inserted in the center comes out clean. Once baked, allow the cake to cool in the pan for 10 minutes before transferring it to a wire rack to cool completely.

While the cake cools, prepare the raspberry mousse. Puree fresh raspberries with granulated sugar in a blender, then strain the mixture through a sieve to remove seeds for a smooth texture. In another bowl, whip heavy cream with vanilla extract until soft peaks form. Carefully fold the melted dark chocolate and raspberry puree into the whipped cream, then refrigerate the mousse for 30 minutes to help it set.

Once the cake is completely cool, slice it in half horizontally to create two layers. Spoon half of the raspberry mousse onto the bottom layer, spreading it gently. Place the top layer back on and cover the entire cake with the remaining mousse. Chill the assembled cake in the refrigerator for at least 1 hour to allow the flavors to meld.

For the ganache, heat heavy cream in a small saucepan until it begins to boil. Pour this hot cream over chopped dark chocolate in a bowl, letting it sit for a minute before stirring until smooth and glossy. Drizzle this ganache over the chilled cake right before serving.