Heat oven to 350'F. Line a 8" or 9" square baking pan with foil, extending edges over sides of pan. In a small mixing bowl, beat butter, 1/2 cup 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 cup sugar until smooth. Add eggs and vanilla; blend well. In separate 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. Drop tablespoonfuls of reserved flavored batter over top, gently swirl with knife or spatula for marbled effect. Bake 35-40 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. About 20 squares.