How to Make It
Preheat oven to 350°F. Grease (with shortening) and lightly flour a 13- x 9-inch pan.
Beat flour and next 8 ingredients at low speed with an electric mixer 30 seconds, scraping down sides as needed. Beat at high speed 2 minutes, scraping down sides as needed. Beat in egg whites and egg at high speed 2 minutes, scraping down sides as needed. Pour batter into prepared pan.
Bake at 350°F for 35 to 45 minutes or until a wooden pick inserted in center comes out clean. Cool in pan on wire rack 10 to 15 minutes; remove from pan to wire rack, and cool completely (about 1 hour).
Make the Pink Lemonade Frosting: Stir 1/4 cup presweetened pink lemonade flavor drink mix in 3 Tbsp. water until dissolved. Beat 1/2 cup softened butter, 1 Tbsp. whipping cream, 1 tsp. lemon zest, and lemonade mixture at low speed with an electric mixer 30 seconds or until creamy. Gradually add 1 (16-oz.) package powdered sugar, beating at low speed until blended. Gradually beat in 1 to 2 Tbsp. whipping cream, 1 tsp. at a time, to make frosting smooth and spreadable. Spread top of cake with frosting.
We tested with Country Time Pink Lemonade Flavor Drink Mix.