White Cake with Berry-Swiss Meringue Buttercream Frosting

Preheat oven to 350°F. Prepare white cake mix according to package directions. Divide batter evenly among 3 (8-inch) round cake pans coated with baking spray.

Bake in preheated oven until a wooden pick inserted in centers of cakes comes out clean, about 20 minutes. Cool in pans 10 minutes. Invert cakes onto wire racks; cool completely, about 45 minutes.

Process raspberries in a food processor until a fine powder forms, about 1 minute. Sift raspberry powder through a fine mesh strainer; discard any unsifted pieces and seeds. Set powder aside.

Beat egg whites, cream of tartar, and kosher salt with a stand mixer fitted with a whisk attachment on medium speed until soft peaks form, about 5 minutes, gradually adding ¼ cup plus 2 tablespoons of the sugar.

Bring ½ cup water and remaining 1½ cups sugar to a boil in a medium saucepan over medium-high. Cook, undisturbed, until mixture reaches 230°F to 240°F (soft-ball stage), about 5 minutes. With mixer on medium speed, gradually add hot sugar mixture to egg white mixture. Increase speed to medium-high, and continue beating until meringue is room temperature, about 10 minutes. Remove whisk attachment from mixer; fit mixer with a paddle attachment. With mixer on medium speed, gradually add softened butter to meringue, 1 tablespoon at a time, beating until butter is combined and mixture is smooth. (If mixture begins to look broken, continue beating until it comes back together.) Beat in vanilla and raspberry powder until combined. Increase mixer speed to high; beat until just fluffy, about 1 minute, stopping to scrape down sides if needed. (You will have about 7½ cups frosting.)

Place a cake plate on a lazy Susan or stand. Put 1 cake layer on plate, and spread top with 1 cup frosting. Repeat with remaining 2 cake layers and 2 cups of the frosting. Using a large offset spatula, spread a thin layer of frosting (about ¾ cup) over sides and top of entire cake. Spoon remaining frosting (about 3¾ cups) into a piping bag fitted with a Wilton 127D petal tip. Hold piping bag so pointed (thin) tip is angled upward. Pipe a small cone-like shape in center of top of cake to create a rosebud. Pipe small arching lines around cone, slightly lifting and dropping your wrist as you pipe and lengthening arches as you work toward outside edges of cake, to create rose petals. Starting at top of cake, pipe 2-inch arches around sides of cake, working your way toward the bottom and overlapping arches slightly, to create petals around cake. See additional step-by-step instructions for frosting a rose design here.