Crush crackers finely into a medium bowl. Melt butter in a large pot over medium-low heat. Add marshmallows and cook, stirring constantly, until melted. Remove from heat and stir in crushed crackers. Working quickly, coat all of the crackers with the marshmallow mixture.

Pour mixture into an 11x7-inch (3 quart) baking dish. Lightly coat the back of a spoon or hands with cooking spray and press mixture firmly and evenly into the baking dish. Alternatively, use a small sheet of wax paper to press the mixture into the pan.