Prep time: 40 min.Decoration time: 30 min.
Line a cookie sheet with waxed paper and top with a wire cooling rack; set aside. Cut pound cake lengthwise into 9 slices, each about 1 inch thick. Using the heart-shaped cookie cutter, cut out 2 heart shapes per cake slice. (Save trimmings for another use, such as trifles, rum balls, or snacking.) Place the heart-shaped cake pieces on top of the wire cooling rack.
Spoon 1/4 cup of the vanilla frosting into a resealable plastic bag and set aside. Using the red food coloring, tint the remaining frosting a light pink, and spoon it into a glass measuring cup. Heat the pink frosting in the microwave for 5 to 10 seconds, or until pourable. DO NOT OVERHEAT. Pour the pink frosting over the heart-shaped cake pieces, to cover both the tops and the sides, reusing the drippings and reheating if necessary. Refrigerate until set, about 30 minutes.
Using scissors, snip a small corner from the resealable plastic bag filled with the white frosting, and use it to pipe designs on the tops and sides of the heart-shaped cake pieces. Attach M&M'S® Brand Milk Chocolate Candies for Valentine's Day in different designs, then pipe messages on the tops with the decorative chocolate frosting.