1. Heat oven to 400°F. Prepare PEANUT BUTTER CRUMB CRUST. Remove wrappers from peanut butter cups. Cut each cup into about 16 pieces.
2. Beat cream cheese and sugar until smooth and creamy in large bowl. Add eggs, beating well. Add sour cream and vanilla; blend thoroughly.
Place 2 cups batter in separate bowl; blend in melted chocolate. Pour chocolate batter into prepared pan. Add peanut butter to remaining batter; blend thoroughly. Gently stir in peanut butter cup pieces; spread over chocolate batter.
3. Bake 10 minutes. Without opening oven door, reduce temperature to 350°F. Bake additional 50 to 55 minutes or until center is almost set.
4. Remove from oven to wire rack. With knife, loosen cake from side of pan. Cool completely. Refrigerate 6 to 8 hours or until thoroughly chilled. Just before serving, garnish with sweetened whipped cream and additional peanut butter cup pieces, if desired. Cover; refrigerate leftover cheesecake. 10 to 12 servings.
PEANUT BUTTER CRUMB CRUST: Stir together 1-1/2 cups vanilla wafer crumbs (about 45 wafers, crushed) and 1/2 cup powdered sugar in medium bowl. Place 1/4 cup REESE'S Creamy Peanut Butter and 3 tablespoons butter or margarine in small microwave-safe bowl. Microwave at MEDIUM (50%) 30 seconds or until butter is melted and mixture is well blended when stirred. Add to crumb mixture, blending thoroughly. Press onto bottom and 1 inch up sides of 9-inch springform pan.