Step 1: Prepare the Chocolate Cake

1. Preheat your oven to 350°F (175°C). Grease and flour an 8-inch round cake pan.
2. In a large bowl, whisk together the flour, cocoa powder, sugar, baking powder, baking soda, and salt.
3. In another bowl, mix the melted butter, eggs, vanilla extract, and buttermilk until well combined.
4. Pour the wet ingredients into the dry ingredients and mix until just combined.
5. Pour the batter into the prepared cake pan and bake for 25–30 minutes, or until a toothpick inserted into the center comes out clean. Let the cake cool in the pan for about 10 minutes, then transfer to a wire rack to cool completely.

Step 2: Make the Raspberry Mousse

1. In a small bowl, sprinkle the gelatin over water and let it sit for about 5 minutes to bloom.
2. Gently heat the bloomed gelatin in the microwave for about 10 seconds until dissolved. Allow it to cool slightly.
3. In a mixing bowl, whip the chilled heavy cream, powdered sugar, and vanilla extract until soft peaks form.
4. In a separate bowl, mash the fresh raspberries and fold them into the whipped cream. Then gently fold in the melted dark chocolate and the dissolved gelatin until fully incorporated.

Step 3: Assemble the Cake

1. Place the cooled chocolate cake on a serving plate. Carefully slice the cake in half horizontally to create two layers.
2. Spread half of the raspberry mousse over the bottom layer of the cake. Place the top layer of the cake on top and spread the remaining mousse over the top layer.
3. Refrigerate the cake for at least 2 hours to allow the mousse to set.

Step 4: Make the Dark Chocolate Ganache

1. In a saucepan, heat the heavy cream until it just begins to simmer. Remove from heat.
2. Add the chopped dark chocolate and butter to the hot cream. Let it sit for a minute, then stir until smooth and glossy.

Step 5: Finish the Cake

1. Once the mousse is set, pour the dark chocolate ganache over the top of the cake, allowing it to drip down the sides.
2. Decorate with fresh raspberries and chocolate shavings if desired.
3. Slice and serve chilled, enjoying the rich flavors of dark chocolate and raspberry.