Combine crumbs and butter; press in the bottom of a greased 13x9 pan. Bake at 350 degrees for 10 minutes. Cool. In a large saucepan over mediun heat, stir in marshmallows and milk until marshmallows are melted. Cool to room temperature. Fold in whipped topping; spread over crust. In a saucepan, bring rasberries, 1 cup of water, sugar and lemon juice to a boil. Combine cornstarch and remaining water; stir into rasberry mixture.Boil 2 minutes, stirring constantly; cool, spread over marshmallow layer. Chill until firm, garnish with whipped cream and rasberries if desired.