Martha Stewart Berry Layer Cake

Directions

• Prep the pans: Preheat the oven to 350°F, then butter three 10-inch round cake pans and line them with parchment. Butter the parchment, dust it with flour and tap out the excess.
• Mix the wet ingredients: Whisk together 1 1/4 cups whole milk, 4 large eggs and the seeds of 1 vanilla bean in a bowl. In a separate bowl, toss 15 ounces blueberries with 1 teaspoon cornstarch.
• Make the batter: Beat 3 cups cake flour, 1 3/4 cups sugar, 1 tablespoon baking powder and 1 teaspoon kosher salt in a mixer with the paddle on low speed. Gradually add 2 sticks butter and beat about 3 minutes until crumbly.
• Add the milk: Slowly add half of the milk mixture, then raise the speed to medium and beat about 2 minutes until fluffy. Add the rest, scraping down the bowl, and beat about 1 minute more.
• Fold in the berries: Take the bowl off the mixer and fold in the cornstarch-coated blueberries. The cornstarch keeps them from sinking to the bottom of the layers.
• Bake the layers: Divide the batter evenly among the pans, spreading to the edges with a small offset spatula, then tap the pans on the counter to release air bubbles. Bake, rotating halfway through, about 30 minutes until a tester comes out clean and the tops are lightly golden.
• Cool the cakes: Set the pans on a wire rack and let them cool completely. Then turn the cakes out and peel off the parchment.
• Warm the meringue: Combine 1 1/2 cups of the 4 1/2 cups sugar, 6 of the 18 egg whites and a pinch of the 3 pinches kosher salt in a mixer bowl set over (not in) simmering water. Whisk 2 to 3 minutes until the mixture is warm and feels smooth between your fingertips.
• Whip to stiff peaks: Move the bowl to the mixer with the whisk attachment and whisk on low until foamy. Then increase to medium-high for about 10 minutes, until the peaks are stiff and glossy and the bowl has cooled completely.
• Beat in butter and vanilla: Reduce to medium-low and add 4 sticks of the 12 sticks butter, 2 tablespoons at a time, whisking until each addition disappears. Whisk in 1 teaspoon of the 3 teaspoons vanilla extract, and don't worry if it looks curdled, since it smooths out with more beating.
• Smooth the buttercream: Switch to the paddle attachment and beat on low 3 to 5 minutes to knock out air bubbles. Transfer to a large bowl, cover with plastic wrap and leave at room temperature.
• Make two more batches: Repeat the buttercream twice more with the same amounts, then combine all three batches in the large bowl and cover. Set aside 2 cups of plain buttercream for lightening the shades later.
• Color the buttercream: Fold 1 1/2 cups black raspberry jam into the large bowl of buttercream. Add burgundy gel food color a drop at a time until you reach the shade you like.
• Assemble the cake: Set one cake layer bottom-side down on a stand and spread about 1 1/2 cups buttercream over it. Top with a second layer, bottom-side down, and another 1 1/2 cups buttercream, then finish with the third layer bottom-side up. Spread a 1/2-inch coat of buttercream over the top and sides, smoothing with an offset spatula.
• Mix the shades: Divide the remaining colored buttercream among three bowls. Then lighten each with a different amount of the reserved white buttercream to make a range of shades.
• Pipe the swirls: Transfer the buttercream to pastry bags fitted with open and closed star tips in a variety of sizes. Pipe swirls and rosettes across the top, then scatter on blueberries, black raspberries and blackberries.