Melt butter in a large saucepan over low heat.
Add marshmallows and stir until melted and well combined. Cook 2 minutes longer, stirring constantly. Remove from heat.
Stir in crispy rice cereal until well coated.
Press mixture evenly and firmly into a buttered 9x13-inch pan using a buttered spatula or waxed paper.
Cut into 2-inch squares when cool.