Three layers of dark chocolate cake, filled with blackberry mascarpone cream and frosted with a fudgy blackberry chocolate buttercream.
Preheat oven to 350 degrees and prepare three 8" cake pans by lining the bottoms with parchment paper. Then, butter and flour the pans.
In a large bowl, whisk together the flour, both sugars, cocoa powder, baking soda, and salt.
Add the eggs, egg yolks, buttermilk, oil, and vanilla. Mix well. Then, pour in the hot coffee and whisk just until combined.
Divide the batter evenly between the three prepared pans and bake at 350 degrees for 30-34 minutes (or until a toothpick inserted into the center of each cake comes out with just a few moist crumbs attached). Let the cakes cool in the pans for 5-10 minutes, then turn each cake out onto a wire rack to cool completely.
Add the blackberries and sugar to a small saucepan over medium-high heat. Stir together and bring the mixture to a boil. Then, reduce the heat to medium and simmer until the juices are released and the mixture is slightly reduced, about 15 minutes. You can use a wooden spoon or spatula to help break down the berries as they cook.
Remove the pan from heat and press the compote through a fine mesh sieve, separating the berry syrup from the mashed blackberries. Reserve both and let cool completely.
Using an electric mixer, beat the butter on medium-high speed until light and fluffy.
Reduce the mixer to low speed and gradually add the cocoa powder, then the powdered sugar, mixing until combined. Finally, add the blackberry syrup a tablespoon at a time until the desired consistency is reached.*** Beat until smooth.
In a medium-size bowl, whisk together the softened mascarpone, powdered sugar, and heavy cream.
Carefully fold in the mashed blackberries. For a swirled pattern, do not incorporate completely.
Once the cakes are completely cool, use a serrated knife or cake leveler to level each cake. Using a small amount of buttercream, secure the first layer to a cardboard cake board.
Fill a pastry bag fitted with a large, round pastry tip with the blackberry chocolate buttercream. Pipe a dam of buttercream all the way around the edge of the first layer of cake, then fill the center with blackberry mascarpone cream. Repeat with the second layer.
Finally, place the third layer on top, cut side down. Frost the top and sides of the cake as desired with the remaining buttercream, then garnish with fresh blackberries. Store finished cake in the refrigerator.
This recipe was adapted from Fork Knife Swoon.
*You can also substitute a scant 1 1/3 cup whole milk mixed with a tablespoon of vinegar or lemon juice.
**For a sweeter filling, you may add more powdered sugar.
***If desired, you may also add a couple tablespoons of the mashed blackberries.