In a blender, combine strawberries, blueberries, blackberries, and raspberries until they form a smooth puree. In a saucepan, mix the berry puree with granulated sugar and vanilla, cooking over medium heat while stirring for about 15-20 minutes until the mixture thickens and coats the back of a spoon. Once thickened, remove from heat and let it cool completely.

Preheat your oven to 350°F (175°C) and grease two 9-inch round cake pans. In a large mixing bowl, whisk together flour, cocoa powder, baking powder, baking soda, and salt until well combined. In another bowl, cream unsalted butter and granulated sugar together until light and fluffy, about 3-5 minutes. Add in the eggs and vanilla, mixing until smooth, then alternately fold in the dry ingredients, buttermilk, and hot coffee until just combined.

Divide the chocolate cake batter evenly between the prepared pans, smoothing the tops. Place the pans in the preheated oven and bake for 39-44 minutes, or until a toothpick inserted into the center comes out clean. After baking, allow the cakes to cool in the pans for 5 minutes, then carefully transfer them to a wire rack to cool completely.

While the cakes cool, prepare the chocolate buttercream. Melt the dark chocolate in a microwave-safe bowl or double boiler until smooth. In a large bowl, beat unsalted butter with a pinch of salt until fluffy. Gradually sift in cocoa powder and powdered sugar, alternating with heavy cream, mixing until the frosting is light and creamy. Finally, add the melted chocolate and whip until fully combined.

Once the cake layers are completely cool, place one layer on a serving plate. Spread a generous layer of chocolate buttercream on top, then add a thick layer of mixed berry jam. Carefully place the second cake layer on top, using additional buttercream to frost the sides and top of the cake for a smooth finish. Chill the assembled cake in the fridge for about 30 minutes to firm up.

To prepare the ganache, heat the heavy cream in a small saucepan until it just begins to simmer. Pour the hot cream over the chopped dark chocolate in a heatproof bowl and let it sit for 2-3 minutes. Stir until the chocolate is fully melted and the mixture is silky smooth. Allow the ganache to cool slightly while holding its pourable consistency.

Once the ganache is at the right temperature and texture, carefully pour it over the top of the chilled cake, allowing it to drip down the sides for a stunning effect. Finally, decorate the top with fresh berries, arranging them artistically for an elegant finish. Your decadent Chocolate Berry Cake is now ready to be sliced and savored!