Prepare cake mix according to package directions. Place batter in 1 square pan and 1 round pan of equal size. Bake according to package directions.
Cool layers; remove from pans. Place square layer on large piece of foil-covered cardboard. Cut round layer in half; join to square layer diagonally, making large heart.
Prepare frosting mix as directed on box; color bright red. Frost cake. Decorate with small valentine candies, if desired.