In large saucepan melt butter over low heat. Add marshmallows and stir until completely melted. Remove from heat.
Add KELLOGG'S® RICE KRISPIES® cereal. Stir until well coated.
Using buttered spatula or wax paper evenly press mixture into 13 x 9 x 2-inch pan coated with cooking spray. Cool. Cut into 2-inch squares. Best if served the same day.
In microwave-safe bowl heat butter and marshmallows on HIGH for 3 minutes, stirring after 2 minutes. Stir until smooth. Follow steps 2 and 3 above. Microwave cooking times may vary.