In a double boiler top over hot, not boiling water, melt chocolate; set aside. (Or, in microwave oven, heat chocolate on high 2 minutes or until shiny. Stir until smooth).
In a large bowl with mixer at medium speed, beat cream cheese and Fluff until smooth. Add eggs and sour cream and beat until blended. Beat in chocolate. Pour into crust and bake 45 minutes or just until edges begin to brown.
Turn off heat and let cheesecake cool in oven with the door cracked open for about 1 hour. Remove to wire rack and cool completely. Cover and refrigerate at least 4 hours before serving.