Heat oven to 350°F. Spray bottom only of 8-inch round cake pan and 8-inch square pan with cooking spray or grease with shortening. 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 2 cups batter into round pan and remaining batter into square pan.
Bake 27 to 32 minutes or until toothpick inserted in center comes out clean. Cool in pans 10 minutes. Remove from pans; place on wire racks. Cool completely, about 30 minutes.
Cut round cake in half. On serving tray or large platter, arrange square cake and round cake halves into heart shape (see diagram); attach cakes with small amount of frosting. Spread frosting on sides and top of cake. Arrange chocolate candies around top edge of cake. About 2 inches inside chocolate candies, arrange cinnamon candies in heart shape.
Place 3-inch heart-shaped open cookie cutter in center of cake. Pour candy decors evenly inside cookie cutter; remove cookie cutter from cake. Arrange conversation heart candies and chocolate candies around smaller heart.