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

In a bowl, whisk together flour, cocoa powder, baking powder, baking soda, and salt.

In a large mixing bowl, beat the softened butter and 1 cup sugar until light and fluffy, about 3–5 minutes; add eggs one at a time, mixing well after each, then stir in 1 teaspoon vanilla extract.

Gradually add the dry ingredients to the butter mixture, alternating with buttermilk, mixing until just combined; pour batter into the prepared pan and smooth the top.

Bake for 25–30 minutes or until a toothpick comes out clean.

Allow the cake to cool in the pan for 10 minutes, then transfer to a wire rack to cool completely; once cooled, slice it in half horizontally to create two layers.

For the raspberry mousse, puree raspberries with 1/4 cup sugar in a blender and strain through a fine mesh sieve.

Whip 1 cup heavy cream and 1 teaspoon vanilla until soft peaks form, then fold in melted and cooled 4 ounces dark chocolate and the raspberry puree; refrigerate about 30 minutes to set.

Place one cake layer on a serving plate, spread half the mousse over it, add the second layer and top with remaining mousse; chill at least 1 hour.

For the ganache, heat 1/2 cup heavy cream until just boiling, pour over chopped dark chocolate and let sit a few minutes before stirring smooth, then drizzle over the chilled cake before serving.