Mix milk, egg yolks, and marshmallows. Heat to custard consistency, cool. Add beaten egg whites and whipped cream. Mas cookies thoroughly and add melted butter to make crumbs. Place all but a couple of Tbls 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. NOTE: Oreo Single filling and Hydrox cookies are about the same. If you can't find Hydrox you can substitute Oreo singles for the Hydrox.