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.