A blueberry-studded triple-decker cake is decorated with swirls and rosettes of frosting flavored with black raspberry jam, followed by a tumble of fresh berries.
Cake: Preheat oven to 350 degrees. Butter three 10-inch round cake pans; line with parchment. Butter parchment, then dust with flour, tapping out excess. Whisk together milk, eggs, and vanilla seeds. In a separate bowl, toss together blueberries and cornstarch.
In the bowl of a mixer fitted with the paddle attachment, beat together flour, sugar, baking powder, and salt on low speed until well combined. Continue beating while gradually adding butter until mixture is crumbly, about 3 minutes.
Slowly add half of milk mixture; increase speed to medium and beat until fluffy, about 2 minutes. Slowly add remaining half of milk mixture, scraping down bowl as needed. Beat until incorporated, about 1 minute more. Remove bowl from mixer; fold in blueberries.
Divide batter evenly between prepared pans, spreading to edges with a small offset spatula. Tap pans on counter to release any air bubbles. Bake, rotating pans halfway through, until a tester inserted in centers comes out clean and tops are lightly golden, about 30 minutes. Transfer pans to a wire rack; let cool completely. Turn cakes out of pans and remove parchment.
Swiss Meringue Buttercream: Combine 1 1/2 cups sugar, 6 egg whites, and a pinch of salt in the heatproof bowl of a mixer set over (not in) a pot of simmering water. Whisk until sugar is dissolved and mixture is warm to the touch and feels completely smooth when rubbed between fingertips, 2 to 3 minutes.
Transfer bowl to mixer fitted with the whisk attachment; whisk on low speed until foamy. Increase speed to medium-high until stiff, glossy peaks form and mixture has cooled completely, about 10 minutes. Reduce speed to medium-low; add 4 sticks butter, 2 tablespoons at a time, whisking until fully incorporated after each addition. (Don't worry if buttercream appears curdled after all of butter has been added; it will become smooth again with beating.) Whisk in 1 teaspoon vanilla.
Switch to paddle attachment. Beat on low speed until air bubbles are eliminated, 3 to 5 minutes. Transfer to a large bowl, cover with plastic, and let stand at room temperature. Repeat process twice more with remaining ingredients, using the same amounts each time, then combining all three batches of buttercream in the large bowl and covering with plastic. Remove 2 cups buttercream; set aside.
Fold jam into large bowl of buttercream. Add burgundy food color, a drop at a time, until desired color is reached.
Assembly:
Place a cake layer, bottom-side down, on a cake stand or serving platter. Spread about 1 1/2 cups buttercream over it; top with a second cake layer, bottom-side down, and another 1 1/2 cups buttercream. Finish with a third cake layer, bottom-side up. Evenly spread top and sides of cake with a 1/2-inch thickness of buttercream, smoothing with an offset spatula.
Divide remaining buttercream among three bowls, then lighten each with some of the reserved 2 cups white buttercream, creating different shades. Transfer to pastry bags fitted with open and closed star tips in a variety of sizes. Pipe swirls and rosettes of buttercream on top of cake. Frosted cake can be refrigerated overnight; bring to room temperature and top with berries before serving.
