Preheat oven to 325 degrees F. Line the inside of an 8 inch square metal pan with non-stick foil.
Process cookies into crumbs (or pound them, use whatever method you like) and mix with butter. Press into the bottom of the pan.
Beat the softened cream cheese, sour cream and sugar until creamy. Add cocoa, cornstarch, egg, egg white and vanilla; beat just until smooth. Pour over cookie crust and sprinkle top with chocolate chips.
Bake on center rack for 35-40 minutes. Cool completely in pan then refrigerate for at least one hour. Invert onto a tray; peel off paper and reinvert crust side down. Cut into 9 squares.