Mix walnuts, flour and margarine together well and press into 9x13 pan. Bake at 325 for 20 minutes. Blend 1/2 cool whip and all powdered sugar and cream cheese and spread over cooled crust. Mix chocolate and vanilla pudding with milk and vanilla and spread over first two layers. Spread rest of cool whip over first 3 layers. Garnish with grated Hershey bar. Cover and refrigerate before serving.