Mix milk, egg yolks, and marshmallows.
Heat to custard consistency, cool.
Add beaten egg whites and whipped cream.
Mash cookies thoroughly and add melted butter to make crumbs.
Place all but a couple of tablespoons of the crumbs on the bottom of a 9 X 13-inch pan.
Spread custard mixture neatly over crumbs.
Sprinkle reserved crumbs over the top.
Chill until firm.