Preheat oven to 325 degrees F (165 degrees C). Grease a 7 1/2-inch square pan and a 7 1/2-inch round pan.
Combine white cake mix, water, 1 1/4 cup strawberries, egg whites, and vegetable oil in a large bowl; mix until well blended. Divide batter between the prepared pans.
Bake in the preheated oven until a toothpick inserted into the center comes out clean, about 50 minutes. Place pans on a wire rack and let cool for 10 minutes.
Flip square cake carefully onto a 12-inch square or round platter, positioning it in a diamond shape. Cut the round cake in half and flip it onto the platter, placing 1 half on each side of the diamond to make a heart shape. Let cool, about 30 minutes.
Combine confectioners' sugar, butter, and 1/2 cup strawberries in a bowl. Beat with an electric mixer until icing is light and fluffy. Ice top and sides of the cake.