In a medium bowl stir together the vanilla wafer crumbs, powdered sugar, cocoa and melted butter. Press firmly onto the bottom of a lightly buttered 9-inch springform pan. Set aside.
Place chocolate chips in microwave-safe bowl. Microwave at 50% power for 1 1/2 minutes; stir. If necessary, microwave at MEDIUM an additional 15 seconds at a time, stirring after each heating, just until chips are melted when stirred.
Using a hand or electric mixer, beat cream cheese in large bowl until fluffy on medium speed. Add sweetened condensed milk and mix until smooth. Add melted chocolate chips, eggs and vanilla; mix well. Pour into prepared crust.
Place springform pan on a cookie sheet and place in oven. Bake at 325 degrees for 60 to 70 minutes or until the very center is almost set. Remove from oven and place on a wire rack. With a knife, carefully loosen cake from side of pan. Cool completely; remove side of pan. Refrigerate several hours before serving. Garnish as desired. Cover; refrigerate leftover cheesecake.