Heat oven to 180ºC . Line 8 or 23 cm square baking pan with foil, extending edges over side of pan. In a small mixing bowl, beat butter, half cup sugar and salt until smooth. Stir together flour and cocoa, gradually add butter to mixture, beating until soft dough is formed. Press dough onto bottom of prepared pan. Beat cream cheese and remaining half cup sugar until smooth. Add eggs and vanilla; blend well. In seperate bowl, mix 1 cup batter with topping, stirring until well blended. Pour 1 cup of topping-flavored batter over dough. Stir melted chocolate into remaining flavored batter; set aside. Gently pour vanilla batter, quarter cup at a time, over flavored batter, Drop tablespoons of reserved flavored batter over 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. Coll completely in pan on wire rackl refrigerate. To serve, lift from pan using foil edges; cut into squares. Garnish as desired. About 20 squares.