Butter a 9×13-inch baking dish.

Melt butter in a large saucepan over low heat; add marshmallows and stir until melted.

Cook and stir the mixture for 2 minutes more. Remove the saucepan from the heat.

Stir crushed cookies into the marshmallow mixture until well coated.

Press cookie mixture firmly into the prepared baking dish using a buttered spatula or waxed paper; cool. Cut into 2-inch squares.