Line a baking sheet with waxed paper. Draw a 9-in. heart on waxed paper. Coat with cooking spray; set aside. In a large saucepan, heat butter and marshmallows over medium heat until marshmallows are melted, stirring constantly. Remove from the heat; stir in food coloring and cereal. Shape mixture into a heart on prepared pan, using the drawn heart as a guide. Let stand for 1 hour or until set.
In a heavy saucepan or microwave, melt chips and shortening; stir until smooth. Spread a 1-in. border near edge of heart. Place remaining melted chips in a pastry or plastic bag; cut a small hole in the corner of bag. Pipe initials in the center of heart.