Who says plain cake has to be, well, plain? Cut pieces from a sheet cake, frost them using a few simple techniques, and watch as your guests -- and their sugar cravings -- delight in their delicious differences.
Preheat oven to 350 degrees. Spray 12-by-17-inch rimmed baking sheet, and line with parchment paper. Spray parchment paper with cooking spray. Sift flour, baking powder, baking soda, and salt into a large bowl.
Beat 3/4 cup sugar and the almond paste in a mixer bowl on medium-low until mixture resembles coarse meal, about 2 minutes. Raise speed to medium-high. Add butter; beat until pale and fluffy, about 2 minutes. Beat in egg yolks and vanilla. Beat in half the flour mixture, then the milk, then remaining flour mixture. Transfer to a large bowl.
Place egg whites in clean mixer bowl; whisk on medium-high until foamy. Gradually add remaining 1/4 cup sugar, beating until soft peaks form. Fold one-third of egg-white mixture into batter with a rubber spatula. Gently fold in remaining egg-white mixture.
Pour batter into prepared pan and spread evenly. Bake until a cake tester inserted into center comes out clean, 15 to 20 minutes. Transfer pan to wire rack; let cool completely. Unmold cake; remove parchment.
Cut cake in half crosswise. Spread half with 2 cups buttercream; top with remaining half of cake. Wrap tightly in plastic; refrigerate 1 hour or up to 2 days. Before decorating, cut into 3-inch squares or circles.
Coat individual cakes with desired covering. Swiss Meringue Buttercream: Use an offset spatula to spread a thin layer of buttercream over each; chill until firm, about 30 minutes.Then coat with a smooth layer (about 1/4 cup each). Place extra buttercream in a pastry bag fitted with a small round tip, and pipe dots along top edges of each. Refrigerate until ready to serve, up to 12 hours. Serve at room temperature.Seven-Minute Frosting: Use an offset spatula to spread a thin layer of buttercream over each; chill until firm, about 30 minutes. Shortly before serving, spread with frosting (1/2 cup each), using an offset spatula to swirl frosting into peaks or to smooth. Let sit at room temperature, up to 4 hours, before serving.Confectioners' sugar: Just before serving, dust about 3 tablespoons over each cake, using a sieve. Serve.Rolled fondant: Use an offset spatula to spread a thin layer of buttercream over each; chill until firm, about 30 minutes. For each, roll out 1-ounce piece of fondant following package directions (wrap remaining fondant airtight). Press a rubber stamp into center. Lay over cake; smooth over sides. Trim excess fondant (if free from crumbs, it can be combined and rerolled). Keep at room temperature until ready to serve, up to 4 hours.