Preheat the oven to 350°F Spray a 9-inch springform pan with vegetable spray.

Make crust: in a bowl, stir together wafer crumbs, water and oil until mixed. Pat mixture onto the bottom and side of the prepared pan.

Make filling: in a food processor, combine ricotta cheese, yogurt, cream cheese, egg, egg white, liqueur,vanilla, sugar and flour; purée until smooth. Pour the mixture into the prepared crust.

Place a pan of hot water on the bottom rack of the over. Bake the cheesecake in the centre of the oven for 50 minutes. The cake will still be quite loose.

Make topping: in a bowl, stir together sour cream, sugar and liqueur. Carefully pour a thin stream of this mixture over top of the hot cheesecake, smoothing it with a knife. Return the cake to the oven and bake for 10 minutes. The topping will be loose and will set as the cake cools.

Run a butter knife around the edge of the cake. Let it cool on a wire rack until it is room temperature. Chill.