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.