Step 1: Make the Chocolate Cake Layer

1. Preheat your oven to 350°F (175°C). Grease and line the bottom of a 9-inch springform pan with parchment paper.
2. In a medium bowl, whisk together the flour, cocoa powder, baking powder, baking soda, and salt.
3. In a large bowl, whisk together the buttermilk, coffee, oil, sugar, eggs, and vanilla extract until smooth.
4. Gradually add the dry ingredients to the wet ingredients, whisking until just combined. Be careful not to overmix.
5. Pour the batter into the prepared pan and smooth the top. Bake for 25-30 minutes, or until a toothpick inserted in the center comes out clean.
6. Let the cake cool in the pan for 10 minutes, then remove it from the pan and let it cool completely on a wire rack.

Step 2: Prepare the Dark Chocolate Mousse

1. Place the chopped dark chocolate in a heatproof bowl. Heat 1/2 cup (120ml) of heavy cream in a small saucepan until it just begins to simmer. Pour the hot cream over the chocolate and let it sit for 2 minutes. Stir until smooth. Let the mixture cool to room temperature.
2. In a separate bowl, whip the remaining 1/2 cup of heavy cream with the sugar until soft peaks form. Gently fold the whipped cream into the cooled chocolate mixture until no streaks remain.

Step 3: Assemble the Cake and Dark Chocolate Mousse

1. Place the cooled cake layer back into the springform pan. Spread the dark chocolate mousse evenly over the cake. Smooth the top with an offset spatula. Refrigerate for 30 minutes to set.

Step 4: Prepare the White Chocolate Mousse

1. Place the chopped white chocolate in a heatproof bowl. Heat 1/2 cup (120ml) of heavy cream in a small saucepan until it just begins to simmer. Pour the hot cream over the white chocolate and let it sit for 2 minutes. Stir until smooth. Let the mixture cool to room temperature.
2. In a separate bowl, whip the remaining 1/2 cup of heavy cream with the vanilla extract until soft peaks form. Gently fold the whipped cream into the cooled white chocolate mixture until no streaks remain.

Step 5: Assemble the Cake and White Chocolate Mousse

1. Spread the white chocolate mousse evenly over the dark chocolate mousse layer. Smooth the top with an offset spatula. Refrigerate the cake for at least 2 hours, or until both mousse layers are fully set.

Step 6: Prepare the Chocolate Ganache

1. Place the chopped dark chocolate in a heatproof bowl. Heat the heavy cream in a small saucepan until it just begins to simmer. Pour the hot cream over the chocolate and let it sit for 2 minutes. Add the butter and stir until smooth and glossy. Let the ganache cool slightly.

Step 7: Assemble the Ganache Layer

1. Pour the ganache over the chilled cake, spreading it evenly across the top. Let the ganache drip slightly down the sides for a decorative effect. Refrigerate the cake for 30 minutes to set the ganache.

Step 8: Serve and Garnish

1. Carefully remove the sides of the springform pan. Transfer the cake to a serving plate.
2. Garnish with shaved chocolate, fresh berries, or dollops of whipped cream as desired.
3. Slice and serve chilled.