1. Heat oven to 350°F. Line 8- or 9-inch square baking pan* with foil, extending edges over sides of pan.
2. Beat butter, 1/2 cup sugar and salt in small bowl until smooth. Stir together flour and cocoa; gradually add to butter mixture, beating on low speed of mixer until soft dough is formed. Press dough onto bottom of prepared pan.
3. Beat cream cheese and remaining 3/4 cup sugar in medium bowl until smooth. Add eggs and vanilla; beat until well blended. Remove 1 cup batter; set aside. Add small chocolate chips to remaining batter; pour over crust.
4. Remove wrappers from chocolate pieces. Place 18 chocolate pieces in medium microwave-safe bowl. Microwave at MEDIUM (50%) 1 minute or until melted when stirred; add to reserved batter, stirring until well blended. Drop by spoonfuls onto batter in pan; gently swirl with knife or spatula for marbled effect.
5. Bake 35 to 40 minutes or until cheesecake is firm and top is slightly puffed. Cool completely in pan on wire rack; refrigerate. To serve, lift from pan using foil edges; cut into squares. Garnish each square with whipped cream and chocolate pieces, if desired. Cover; refrigerate leftover cheesecake. Makes 16 squares or 12 wedges.
* 8 or 9-inch springform pan can also be used.