Heat oven to 350°F (325°F for dark or nonstick pans). Grease or lightly spray bottoms only of one 8-inch round and one 8-inch square pan.
In large bowl, beat cake mix, water, oil and egg whites with electric mixer on low speed 30 seconds, then on medium speed 2 minutes, scraping bowl occasionally.
Pour batter into pans.
Bake square pan 25 to 29 minutes, round pan 29 to 34 minutes or until toothpick inserted in center comes out clean. Cool 10 minutes; remove from pans. Cool completely, about 1 hour.
Cut round cake in half. Put square cake on tray with one point toward you.
Place cut side of each half against one of the top sides of square cake to make a heart. (See link below for diagram.)
Tint frosting with a few drops food color. Frost cake with frosting. Store loosely covered.
