Makes: one 9-inch layer cake
3 1⁄2 cups flour, plus more for pans
1 tablespoon plus 1 teaspoon baking powder
1⁄2 teaspoon salt
1 cup milk
1 1⁄2 teaspoons vanilla extract
2 sticks (1⁄2 pound) butter, plus more for greasing pans
2 cups sugar, divided
8 egg whites
1 1⁄2 cups egg whites (about 12)
3 cups sugar
3 pounds butter, cut into small pieces
2 tablespoons vanilla extract
3⁄4 cup raspberry preserves
1 to 1 1⁄2 pints fresh raspberries, plus more for garnish
For the cake: Preheat oven to 375°F. Butter two round 9×2-inch cake pans and line with parchment paper. Butter the paper and dust with flour.
In a bowl, stir together flour, baking powder, and salt. In a measuring cup, combine milk and vanilla.
In a mixer fitted with the paddle attachment, cream butter and 1 2∕3 cups sugar until fluffy. Alternating, add flour mixture and milk mixture to butter, adding about one-third at a time. Mix until just blended.
In another clean bowl, with a mixer, beat egg whites until frothy. Gradually beat in remaining 1∕3 cup sugar and beat until stiff peaks form. Fold whites into batter.
Divide batter evenly between cake pans. Bake until a wooden pick comes out clean. Cool completely.
For the buttercream: In a saucepan, bring an inch or two of water to a boil over high heat. In a mixing bowl (that will fit over the pan without touching water), whisk egg whites and sugar to combine. Reduce heat under the water to medium and place bowl over the pan. Cook, whisking every few minutes. Let mixture heat through and melt sugar (you won’t feel sugar granules anymore).
Remove from heat and, with using whisk attachment, beat with mixer until pale and fluffy and no longer hot. Slowly beat in butter, a tablespoon at a time; let fully combine after each addition. Add vanilla and whisk until combined.
To assemble, with a long serrated knife, horizontally slice both cake rounds in half (along the equator). Place a cake round on a serving plate. Spread with 1⁄4 cup raspberry preserves and cover with 1⁄2 cup frosting. Repeat for 2 more cake layers. Top with final layer. Using a small palette knife or spreader, coat the top and sides of the cake with a layer of frosting—about 1⁄2 inch deep.
Fill a piping bag fitted with a large leaf tip with frosting. Starting at the bottom edge of the cake, and holding the thicker edge of the tip against the cake, pipe a ribbon. Let it ripple a bit at the bottom, and straighten out toward the top. At the top edge, let it make a little peak above the top of the cake. Continue around the cake.
Fill the top with fresh raspberries. Chill until ready to serve, then garnish the base of the cake with more fresh raspberries.