1. Stir together graham cracker crumbs, melted butter and 1/4 cup sugar in small bowl; pat firmly onto bottom of 9-inch springform pan.
2. Place chocolate chips in small microwave-safe bowl. Microwave at MEDIUM (50%) 1 minute or just until chips are melted when stirred.
3. Beat cream cheese and remaining 1-3/4 cups sugar in large bowl; add eggs, one at a time, beating after each addition. Stir in sour cream and orange extract. Stir 3 cups cream cheese mixture into melted chocolate chips; pour into crust. Freeze 10 to 15 minutes until chocolate sets.
4. Heat oven to 325°F. Stir orange peel into remaining cream cheese mixture; gently spread over chocolate mixture.
5. Bake 1 hour 15 minutes or until set except for 3-inch circle in center; turn off oven. Let stand in oven, with door ajar, 1 hour; remove from oven. With knife, loosen cheesecake from side of pan. Cool completely; remove side of pan. Cover; refrigerate. Garnish with whipped topping and orange wedges, if desired. Cover; refrigerate leftover cheesecake. Makes 12 servings.