Preheat your oven to 350°F (175°C) and lightly grease and flour an 8-inch round cake pan. In a large mixing bowl, combine flour, cocoa powder, baking powder, baking soda, and salt. In a separate bowl, beat the softened butter and granulated sugar until fluffy and light. Add eggs one at a time, followed by vanilla extract. Gradually mix in the dry ingredients alternating with buttermilk until just combined. Pour the batter into the prepared pan and bake for 25-30 minutes, or until a toothpick inserted comes out clean. Allow the cake to cool completely.

While the cake cools, prepare the raspberry mousse. In a blender, puree fresh raspberries with granulated sugar, then strain the mixture through a fine-mesh sieve to remove seeds. In another bowl, whip heavy cream and vanilla extract until you achieve soft peaks. Gently fold the cooled melted dark chocolate and raspberry puree into the whipped cream until evenly incorporated. Chill the mousse in the refrigerator for about 30 minutes to set.

Once the cake has cooled entirely, slice it horizontally into two equal layers. Place one layer on a serving plate and generously spread half of the raspberry mousse over the top, ensuring even coverage. Carefully place the second cake layer on top, then spread the remaining mousse over the cake's surface and sides. Chill the assembled Dark Chocolate Raspberry Mousse Cake for at least one hour to allow the flavors to meld.

To make the ganache, heat the heavy cream in a small saucepan until it just begins to simmer. Remove from heat and pour it over chopped dark chocolate in a mixing bowl. Let it sit for a few minutes to soften, then stir until smooth and shiny. Once cooled slightly, drizzle it over the chilled cake.

When ready to serve, garnish the cake with fresh raspberries and a sprinkle of powdered sugar. Slice into the Dark Chocolate Raspberry Mousse Cake to enjoy this delightful combination of rich chocolate and vibrant raspberry flavors.