Spray a 9x13-inch pan with nonstick spray and set aside.

Place 3 tablespoons butter and the entire 10-ounce package of marshmallows in a large microwave-safe bowl.

Microwave on HIGH for 3 minutes, stopping once halfway through to stir. After 3 minutes, stir until the marshmallows and butter are melted and smooth; if small pieces remain, microwave an additional 15–20 seconds and stir again.

While the marshmallows are heating, combine 6 cups Rice Krispy cereal, 18 crushed Nutter Butter cookies, and the broken chunks from the 2 large movie-size Hershey bars in a large mixing bowl.

Pour the melted marshmallow mixture over the cereal/cookie/chocolate mixture. Use a spatula lightly greased with a little of the melted butter (or lightly buttered hands) to fold and stir until everything is evenly coated.

Transfer the mixture to the prepared 9x13 pan. Press it firmly and evenly into the pan, working it into the corners with the greased spatula or lightly buttered hands.

Let the pan cool at room temperature until the treats are set and firm (about 30–60 minutes).

Cut into squares and serve. Best eaten the same day as making.