1. Heat oven to 350°F. Beat butter, 1/2 cup sugar and salt in small bowl until smooth. Stir together flour and cocoa; gradually add to butter mixture, beating until soft dough forms. Press dough onto bottom of 8- or 9-inch springform pan.
2. 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 1/2 cup small chocolate chips to remaining batter; pour over crust.
3. Place remaining chips in medium microwave-safe bowl. Microwave at MEDIUM (50%) 1 minute; stir. Microwave at MEDIUM in 15 second increments until chocolate is melted and smooth when stirred. Gradually add to reserved batter, blending well. Drop by spoonfuls onto batter in pan; gently swirl with knife for marbled effect.
4. Bake 35 to 40 minutes until cheesecake is firm and top is slightly puffed. With knife, immediately loosen cake from side of pan. Cool completely. Cover; refrigerate until chilled. Garnish as desired. Cover; refrigerate leftover cheesecake. Makes 12 servings.