Heat oven to 180ºC . Line 20 cm square baking pan with foil, extending edges over sides of pan. In a small mixing bowl, beat butter, half c. sugar and salt until smooth. Stir together flour and cocoa; gradually add to butter mixture, beating until soft dough is formed. Press dough onto bottom of prepared pan. Beat cream cheese and remaining
1./2 c. of sugar until smooth. Add eggs and vanilla; blend well. In separate bowl, mix 1 cup batter with topping, stirring wntil well blended. Pour 1 c. of topping-flavored batter over dough. Stir melted chocolate into remaining flavored batter; set aside. Gently pour vanilla batter, quarter c. at a time, over flavored batter. Drop tablespoonsful of reserved flavored batter ovver top; gently swirl with knife or spatula for marbled effect. 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 as desired. Yield: about 20 squares