Line two 9-in. round baking pans with waxed paper. Coat pans with cooking spray and sprinkle with flour; set aside. In a large bowl, combine cake mix and gelatin. Add egg whites and oil; beat until well blended.
In a small bowl, mash strawberries in their juice. Set aside 3 tablespoons for icing. Add water and remaining berries to the batter; mix well.
Pour into prepared pans. Bake at 350° for 20-25 minutes or until a toothpick inserted in the center comes out clean. Cool for 10 minutes before removing from pans to wire racks to cool completely.
For icing, in a small bowl, combine butter and reserved strawberries. Gradually beat in confectioners' sugar until light and fluffy. Place one cake layer on a serving platter; top with half of the icing. Repeat layers.