Make the Cake:
Preheat oven to 350°F (175°C). Grease and flour an 8-inch round cake pan.
In a bowl, whisk together flour, cocoa powder, baking powder, baking soda, and salt.
In another bowl, beat butter and sugar until light and fluffy (3–5 minutes).
Add eggs one at a time, mixing well, then add vanilla extract.
Alternate adding dry ingredients and buttermilk to the batter. Mix until just combined.
Pour into pan and bake for 25–30 minutes or until a toothpick comes out clean.
Cool in pan for 10 minutes, then transfer to a wire rack to cool completely.

Prepare Raspberry Mousse:
Blend raspberries with sugar and strain through a fine-mesh sieve to remove seeds.
Whip heavy cream and vanilla extract until soft peaks form.
Fold in melted dark chocolate and raspberry puree until smooth.
Chill mousse for 30 minutes to firm up slightly.

Assemble the Cake:
Slice the cooled cake in half horizontally to create two layers.
Place the bottom layer on a serving plate and spread half the mousse evenly.
Top with the second cake layer and spread remaining mousse on top.
Chill the entire cake for at least 1 hour.

Make Ganache and Finish:
Heat heavy cream until just boiling, then pour over chopped chocolate. Let sit for 2–3 minutes, then stir until smooth.
Drizzle ganache over the chilled cake. Garnish with fresh raspberries if desired.