Melt butter in stock pot on low heat. Add marshmallows and stir constantly until they melt. Turn off heat, leave pot on burner and stir in cereal gradually until thoroughly mixed. Spray 13x9 inch pan with vegetable spray and pour batter into pan and press batter firmly throughout pan using a vegetable sprayed spatula. Let cool before cutting into 2 inch pieces. Makes 24 pieces, one piece is one serving.