Preheat oven to 400°.
Combine first 4 ingredients in a bowl, and toss with a fork until blended. Press crumb mixture into the bottom of a 9-inch springform pan coated with cooking spray. Bake at 400° for 8 minutes. Cool crust on a wire rack. Increase oven temperature to 525°.
Combine semisweet chocolate and milk in a bowl; microwave at high 45 seconds or until chocolate melts, stirring after 30 seconds. Cool.
Combine 1 1/4 cups sugar, cheeses, vanilla, and salt in a food processor, and process just until smooth. Add egg whites, and process until blended. Add chocolate mixture, cocoa, fudge topping, and sour cream, and process until blended.
Spoon batter into prepared pan. Bake at 525° for 7 minutes. Reduce oven temperature to 250°, and bake 25 minutes or until almost set. Cheesecake is done when the center barely moves when the pan is touched. Remove cheesecake from oven; run a knife around outside edge, and cool to room temperature. Cover and chill at least 8 hours.