Tint 1/2 cup frosting pink, transfer to freezer-weight plastic bag, and cut off small corner. Cut the round cake in half. Place the square cake on a large serving platter. Frost the cut edges of the round cake, then place the two halves at top corners of the square cake to create a heart shape. Frost cake with the remaining white frosting. Pipe dots of pink frosting on cake. Top with sprinkles.