Butter a 9x13-inch pan; set aside. Melt 1/4 cup butter in a large saucepan over low heat.

Add marshmallows; stir until melted and well combined. Cook for 2 minutes more, stirring constantly. Remove from heat.

Stir in crispy rice cereal until well coated.

Press mixture evenly and firmly into the prepared pan using a buttered spatula or waxed paper.

Cool, then cut into 2-inch squares.