Preheat the oven to 350 degrees F. Butter 2 (8-inch) round cake pans and line the bottoms with parchment paper or waxed paper.
Prepare the cake batter according to the package directions. To the batter, stir in the lemon zest, vanilla, and pink lemonade powder. Pour the batter evenly into the prepared pans. Bake until golden and a toothpick inserted into the center of the cake comes out clean, 30 to 35 minutes. Let the cakes cool in the pans for 10 minutes. Carefully turn the cakes out onto a wire rack to cool completely.
While the cake is baking, prepare the frosting by beating together the confectioners' sugar and remaining 1 cup butter until fluffy. Beat in the remaining frosting ingredients until combined.
Transfer one cake to a cake stand or large platter. Using an offset spatula, spread the top of the cake with a layer of frosting. Place the second cake on top of the first. Spread the remaining frosting over the top and sides of both layers.