Line a 9x13x2-inch baking pan with foil, extending the foil over the edges of the pan. Lightly coat foil with nonstick cooking spray; set aside.
Melt the margarine and marshmallows in a 4-to 5-quart Dutch oven over low heat, stirring frequently. When the marshmallows are melted and smooth, pour in the cranberries and cereal. Mix well with a wooden spoon.
Press mixture evenly into the pan using slightly wet hands. Let cool. Use a heart-shape cookie cutter to cut. Wrap individually in plastic wrap to store.