1¾ cups (219 grams) all-purpose flour1 cup (200 grams) granulated sugar2 teaspoons (10 grams) baking powder½ teaspoon kosher salt½ cup (113 grams) unsalted butter, softened½ cup (120 grams) whole milk, room temperature2 large eggs (100 grams), room temperature1½ teaspoons (6 grams) vanilla extract½ cup (85 grams) rainbow jimmiesAlmond Swiss Meringue Buttercream (recipe follows)Garnish: rainbow jimmies, white nonpareils
Preheat oven to 325°F (170°C). Lightly spray 2 (6-inch) round cake pans with baking spray with flour. Line bottom of pans with parchment paper.
In the bowl of a stand mixer, whisk together flour, sugar, baking powder, and salt by hand. Add butter; using the paddle attachment, beat at low speed until butter is incorporated and mixture resembles coarse crumbs.
In a medium bowl, whisk together milk, eggs, and vanilla. With mixer on low speed, add about half of milk mixture to flour mixture. Slowly increase mixer speed to medium, and beat until mixture is light and fluffy, 2 to 3 minutes, stopping to scrape bottom and sides of bowl. Reduce mixer speed to low, and gradually add remaining milk mixture, beating until combined and stopping to scrape bottom and sides of bowl. Fold in jimmies until combined. Divide batter between prepared pans (about 1¾ cups or 415 grams each). Using a small offset spatula, spread batter into an even layer.
Bake until golden brown, a wooden pick inserted in center comes out clean, and an instant-read thermometer inserted in center registers 200°F (93°C) to 205°F (96°C), 40 to 45 minutes. Let cool in pans for 10 minutes. Remove from pans, and let cool completely on a wire rack.
Level cooled cake layers, if desired. Place 1 cake layer on a cake board or serving platter. Spread ¾ cup (140 grams) Almond Swiss Meringue Buttercream onto cake layer, and top with remaining cake layer. Spread remaining buttercream on top and sides of cake, and garnish with jimmies and nonpareils, if desired. Refrigerate cake for 30 minutes before serving. If refrigerating for longer than 1 hour, let cake stand at room temperature for 20 to 30 minutes before serving. Refrigerate in an airtight container for up to 3 days.