COMBINE crumbs and margarine (setting aside 1 Tbsp. of crumbs for topping); press onto bottom of foil-lined 9x5-inch loaf pan.

MIX cream cheese, peanut butter, sugar and vanilla with electric mixer on medium speed until well blended. Gently stir in the whipped topping. Spoon 1/2 cup of the cream cheese mixture into small bowl. Stir in melted chocolate until well blended; set aside. Spoon half of the remaining cream cheese mixture over crust. Top evenly with chocolate mixture; cover with remaining cream cheese mixture.

FREEZE 4 hours or overnight until firm. Invert onto plate. Remove foil, then re-invert onto serving platter so that crumb layer is on bottom. Sprinkle with reserved 1 Tbsp. crumbs.