This show-stopping cake recipe comes from Chef Tiffany MacIsaac of Buttercream Bake Shop in Washington D.C.
Preheat oven to 325°F. Coat 3 (8-inch) round cake pans with cooking spray and line bottoms with parchment paper.
Place sugar, butter, shortening, and lemon zest in the bowl of a heavy-duty electric mixer fitted with the paddle attachment. Beat at medium speed until well combined and smooth, 1 to 2 minutes, scraping sides of bowl as needed. Add eggs and egg yolk, one at a time, beating on medium speed just until combined after each addition. Beat in vanilla. Scrape sides of bowl and beat on medium speed until well combined, 5 to 10 seconds.
Whisk together flour, baking powder, and salt in a medium bowl. In a separate bowl, combine buttermilk and lemon extract. Add half of flour mixture to batter and beat on low speed just until combined. With mixer running, gradually add buttermilk mixture. Add remaining flour mixture and beat until combined. Increase speed to medium and beat until smooth, about 15 seconds. Divide batter among prepared pans (about 2 1/4 cups per pan).
Bake in preheated oven until golden brown a toothpick inserted in center comes out clean, 25 to 35 minutes. Cool in pans 15 minutes; remove from pans and cool completely on wire racks, about 1 hour. Wrap cake layers individually in plastic wrap and refrigerate 2 hours.
Use a long serrated knife to trim tops of cake layers so that they are flat. Place one layer, cut side up, on a cake plate or board at least 10 inches in diameter. Pipe a ring of Vanilla Buttercream around top edge of cake layer. Spoon half of Lemon Curd in center and spread evenly. Top with a second cake layer, cut side up, and repeat with Vanilla Buttercream and remaining Lemon Curd. Top with third cake layer, cut side down. Spread a thin layer of buttercream over top and sides of cake. Refrigerate at least 2 hours or up to overnight.
Prepare Honey-Vanilla Meringue and immediately spread over top and sides of cake. Use a kitchen torch to toast meringue until golden brown. Sprinkle with edible glitter, if desired. Serve immediately or cover and refrigerate up to 2 days.