Mix crumbs and butter; press firmly onto bottom of 9" spring form pan. Refrigerate for 10 min. Beat cream cheese, peanut butter and sugar with electric beater until well-blended. Chop 1 bar of the chocolate; stir into cream cheese mixture. Gently stir in 1 cup of whipped topping. Spoon over crust. Refrigerate for 3 hours. Microwave remaining 1/2 cup whipped topping and one bar on high. Stir until chocolate is molten & mixture well-blended. Cool slightly and pour over cake. Refrigerate until ready to serve.