Preheat oven to 350°F. For the Cake, grease and flour 1 each 9-inch round and 9-inch square cake pan. Set aside. Beat cake mix, sour cream, water, cocoa powder, oil, food color, eggs and vanilla in large bowl with electric mixer on low speed just until moistened, scraping sides of bowl frequently. Beat on medium speed 2 minutes. Pour batter into prepared pans.
Bake 30 minutes or until toothpick inserted in center comes out clean. Cool in pan 10 minutes. Remove from pans; cool completely on wire rack.
For the Frosting, beat cream cheese, butter, sour cream and vanilla in large bowl until light and fluffy. Gradually beat in confectioners’ sugar until smooth.
Cut round cake in half into 2 half-moons. Place square cake on serving platter. Spread small amount of frosting on the cut ends of each half-moon. Press onto 2 adjacent sides of the square cake to form a heart shape. Frost top and sides of cake with remaining frosting. Garnish with sprinkles, if desired.