How to Make It
Prepare the Cake: Preheat oven to 350°F. Coat a 17- x 12-inch half-sheet pan with cooking spray, and line bottom of pan with parchment paper. Sift cake flour, baking powder, and salt together in a medium bowl. Set aside.
Beat butter and oil in a large bowl with an electric mixer on medium-high speed until smooth and creamy. Add sugar, and beat until light and fluffy, 4 to 5 minutes. Add eggs, 1 at a time, and beat on medium-low speed until well blended after each addition. Beat in vanilla. Add flour mixture, one-third at a time, to butter mixture alternately with buttermilk, beginning and ending with flour mixture. Beat on low just until smooth after each addition. (Do not overbeat or cake will be tough.) Spread batter evenly in prepared sheet pan. Gently tap pan on countertop to release any air bubbles.
Bake in center of preheated oven until a wooden pick or cake tester inserted in center of cake comes out clean, 18 to 22 minutes. Place pan on a wire rack.
Heat thawed limeade concentrate in a small saucepan over medium just until warmed through, about 3 minutes. Brush cake lightly with about half of the limeade syrup. (Reserve remaining limeade syrupfor later use.) Cool cake completely in pan, about 45 minutes. Freeze cooled cake, uncovered, until very cold and very firm, about 30 minutes.
Meanwhile, prepare the Buttercream. Beat powdered sugar and butter in a large bowl with an electric mixer on medium-high speed until thick and creamy, 4 to 5 minutes. Beat in vanilla. Add mascarpone cheese, and beat on low speed just until combined, stopping to scrape down sides of bowl as needed. (Do not overmix or the mascarpone will start to break down and the frosting will become too thin.)
Fit a large disposable piping bag with a large star tip. Fill piping bag with 3 cups of the Buttercream. Remove cake from freezer. Using a 2 3/4-inch round cutter, carefully cut 16 circles from sheet cake. Use an offset spatula to transfer cake circles to another sheet pan. Discard cake scraps or reserve for another use.
Brush top and sides of each cake circle with reserved remaining limeade syrup. Pipe a tight spiral of icing on 8 of the cake layers. Top each frosted layer with unfrosted cake layer to make 8 miniature 2-layer cakes. Hold 1 cake on your fingertips, and, using an offset spatula, carefully swipe sides of cake with a thin layer of Buttercream, barely covering the cake. Repeat with remaining cakes. Pipe a rosette of Buttercream on top of each cake. Garnish with white chocolate curls, white nonpareils, a fresh berry, or edible flower.
Keep cakes covered and refrigerated until almost party time. Then let stand at room temperature 30 minutes before enjoying.