Butter a 9x13-inch baking dish.

Melt butter in a large saucepan over low heat; add marshmallows and stir until melted. Cook and stir mixture for 2 minutes more. Remove saucepan from heat. Stir crushed cookies into 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.