How to Make It
Preheat oven to 350°F. Stir together all wet ingredients needed to prepare cake batters (per package directions) in 1 large bowl. Stir together all dry ingredients from cake mix packages in a separate bowl. Add dry ingredients to wet ingredients; stir together until well blended. Divide combined batter among 3 greased and floured 9-inch round cake pans. Bake until a wooden pick inserted in center comes out clean, 25 to 30 minutes. Cool cake layers in pans on a wire rack 10 minutes. Invert cake layers onto racks, and cool completely, about 45 minutes.
Prepare Lemon-Vanilla Buttercream.
Using a serrated knife, trim domed tops of cake layers to make them flat and even. Spread Lemon-Vanilla Buttercream between layers and on top and sides of cake, allowing some of the cake to remain visible on the sides. Smooth with an offset spatula. Arrange a ring of edible flowers and mint leaves around top edge of cake to form a “flower crown.”