Preheat your oven to 350°F (175°C). Grease and flour an 8-inch round cake pan.
In a medium bowl, whisk together flour, cocoa powder, baking powder, baking soda, and salt. Set aside.
In a large bowl, beat the softened butter and granulated sugar until light and fluffy, about 3-5 minutes.
Add the eggs one at a time, mixing well after each addition. Mix in the vanilla extract.
Gradually add the dry ingredients to the butter mixture, alternating with buttermilk, starting and ending with the dry ingredients. Mix until just combined.
Gently fold in the fresh raspberries into the batter.
Pour the batter into the prepared cake pan and smooth the top. Bake for 25-30 minutes, or until a toothpick inserted in the center comes out clean. Let it cool in the pan for 10 minutes, then turn it out onto a wire rack to cool completely.
While the cake is cooling, melt the dark chocolate in a double boiler or microwave until smooth. Allow it to cool slightly.
In a separate bowl, whip the heavy cream and powdered sugar until soft peaks form.
Fold the melted chocolate into the whipped cream until fully combined and smooth. Be careful not to deflate the whipped cream.
Once the cake is completely cooled, slice the cake into two layers horizontally. Spread half of the chocolate mousse on the bottom layer, then place the top layer back on. Spread the remaining mousse over the top and sides of the cake.
Refrigerate the cake for at least 2 hours to let the mousse set.
Before serving, garnish with fresh raspberries and mint leaves.
Notes
Chill the cake for at least 2 hours before serving for best results.