Preheat the oven to 180 degrees C. Grease the sides of a 22cm springform pan.
Crust: In a medium bowl mix together the biscuit crumbs, ground walnuts and melted butter. Press into the bottom of the prepared pan.
Filling: In a large bowl stir cream cheese to soften. Mix together sugar and flour then stir into the cream cheese until smooth. Stir in eggs one at a time mixing until well blended after each one.
Stir in sour cream, mashed banana, banana liqueur and vanilla. Pour over the crust in the springform pan.
Cover the bottom of the outside of the cheesecake pan with aluminium foil to prevent water from the water bath seeping in. Place springform pan inside a larger pan. Place the whole thing into the preheated oven and fill the outer pan with hot water.
Bake for 45 minutes in the preheated oven. After the time is up turn the oven off but leave door closed. Leave cheesecake in the unopened oven for 1 hour.
Before removing from the water bath. Run a knife around the outer edge of the cake to keep it from shrinking away from the centre and cracking.
Let cool to room temperature then refrigerate until chilled at least 3 hours or overnight.
Topping: In a small bowl sprinkle the gelatine over the cold water and set aside to soften.
Heat milk in a small saucepan over medium-low heat until hot but not boiling. Meanwhile whisk together sugar and egg yolks until smooth and frothy. Whisk about 1/3 of the hot milk into the egg yolk mixture then pour the yolk mixture into the saucepan with the remaining milk.
Cook over low heat stirring constantly with a spatula making sure that the mixture does not burn on the bottom until it is thick enough to coat the back of a metal spoon.
Remove from the heat. Stir the softened gelatine into the hot pastry cream until dissolved then stir in vanilla. Pour into a bowl place a sheet of plastic wrap directly on the surface and refrigerate until cooled; about 1 hour.
When the pastry cream is cooled wisk the cream just past soft peaks. Stir pastry cream to soften then fold in the whipped cream.
Place vanilla wafers on top of the cooled cheesecake then spread the vanilla cream over the entire top. Chill until serving.
Run a wet knife around the outer edge of the cake before removing the sides for a cleaner look.