Preheat oven to 180ºC . Stir together wafer crumbs and cinnamon. Pat into bottom of 23 cm springform pan. Beat cream cheese until light and fluffy. Beat in sugar and cocoa powder. Beat in egg. Stir in 2 cups sour cream, coffee liqueur and vanilla. Turn into prepared pan. Bake for 30 minutes or until set. Spread remaining sour cream evenly over top. Return to oven 1 minute to glaze top. Cool to room temperature, then chill thoroughly, covered. Remove from springform pan. Just before serving, dust top with cocoa powder.