4 large eggs, at room temperature
2 large egg yolks, at room temperature
1 1/4 buttermilk, shaken
2 teaspoons vanilla extract
3 cups cake flour, sifted (I used Softasilk)
2 cups granulated sugar
1 tablespoon plus 1/2 teaspoon baking powder
1 teaspoon baking soda
1/2 teaspoon salt
1 cup (2 sticks) unsalted butter, cold, and diced into small pieces
For the Strawberry-Blackberry Compote
1 heaping cup fresh strawberries, washed, hulled, and sliced
1 heaping cup fresh blackberries, washed
1/3 cup granulated sugar
1 tablespoon lemon juice
pinch of salt
For the Whipped Masarpone Filling
2 tablespoons cold water
2 teaspoons unflavored gelatin
1 3/4 cup heavy cream, cold
1/4 cup confectioner’s sugar
1 teaspoon vanilla extract
1/2 cup mascarpone cheese, softened
pinch of salt
For the Whipped Vanilla Frosting
3 sticks + 2 tablespoons unsalted butter, at room temperature
3 1/2 cups confectioner’s sugar, sifted
3 tablespoons milk
1 tablespoon vanilla extract
pinch of salt
Begin by preparing the buttermilk cake. Preheat the oven to 350 degrees F, and position a rack in the middle of the oven. Lightly spray 3 8-inch round cake pans with nonstick spray and line the bottoms with wax paper. Lightly spray the pans again and flour, making sure to tap out the excess.
In a large measuring cup, whisk together the eggs, egg yolks, 1/4 cup of the buttermilk, and vanilla extract. Set aside.
In the bowl of an electric mixer fitted with the paddle attachment, stir together the flour, sugar, baking powder, baking soda, and salt on low speed, just to combine. With the mixer running on low speed, add the cold butter to the mixer bowl, one piece at a time, about 10 seconds apart. It seems a little tedious, but trust me, this works. Once all the butter has been added, continue mixing until the butter has been blended and there are no clumps, about another minute and a half to two minutes. The mixture will have a very fine, crumbly, cornmeal-like texture.
Add the remaining 1 cup of buttermilk to the dry ingredients and mix on medium speed for 4 minutes, making sure to scrape down the sides and bottom of the bowl at the end to ensure that all the ingredients were incorporated. Reduce the mixer speed to low, and slowly pour in the egg mixture; once the egg mixture has been added, increase the speed to medium and beat for 1 more minute. Gently fold the batter once or twice to ensure that the egg mixture has been fully incorporated.
Divide the cake batter evenly among the three prepared pans (a kitchen scale is tremendously helpful here). Place two of the cake pans in the oven, and bake until a toothpick inserted into the center comes out clean , about 30-32 minutes. Bake the final layer. Set the cakes on a wire rack and let them cool for 10 minutes. Use an offset spatula to loosen the cakes from the baking pans and then invert them onto the wire racks. Carefully peel off the wax paper liner, and let the cakes cool completely.
While the cakes are cooling, prepare the berry compote. In a small saucepan, combine 3/4 cup of the sliced strawberries, 3/4 cup blackberries, sugar, lemon juice, and pinch of salt. Set the saucepan over medium heat, and bring to a simmer for about 10 minutes, letting the berries break down. Lower the heat, and let the berries simmer for an additional ten minutes, until their syrup coats the back of a wooden spoon. Remove the compote from the heat, and let cool to room temperature. With an immersion blender or hand masher, gently mash the berries a bit. Stir in the remaining fresh berries, cover, and refrigerate until ready to use. The berry compote can easily be made the day before you need it.