Preheat oven to 300. Prepare crust. Stir together butter & cocoa until smooth. In large mixer bowl, beat cheese until fluffy. Gradually beat in condensed milk until smooth. Beat in eggs & vanilla then flour. Measure 1 1/2 cups batter into medium bowl. Beat in cocoa mixture. Spoon half the vanilla batter into prepared shell then half the chocolate batter. Repeat, ending with chocolate batter. With metal spatula, cut through batters to marble. Bake 50-60 minutes or until center is set. Cool. Refrigerate leftovers. Chocolate crumb crust: Combine 1 1/2 cups vanilla wafers (about 45 wafers), 1/2 cup confectioners sugar, 1/3 cup cocoa & 1/3 cup melted butter. Press firmly on bottom of 9" springform pan.