Place sugar and vanilla bean into a food processor and mill until fine.
Add remaining pastry ingredients except sour cream and mix until a crumb forms.
Add sour cream and continue mixing until you have a ball of dough.
Tip dough out onto floured silpat mat and push into a disc. Refrigerate 15-30 minutes.
Roll out pastry and line a 24 cm flan tin with removable base with the pastry. Place into the freezer while you prepare the filling.
Preheat oven to 160°C.
Melt the chocolate in the microwave and set aside.16
Place all remaining filling ingredients except the walnuts into a food processor and blend well. Stir though the walnuts.
Brush the pastry base with the melted chocolate. Pour filling over. Bake 45-50 minutes until just set in the centre. Cool completely before topping with the cream.
To make the bourbon cream, place 400ml cream, 1 tsp vanilla bean paste, 3 tsp icing sugar and 20ml bourbon into a stand mixer and whip until soft peaks form. Top the pie with this cream in billowy mounds. Garnish with grated dark chocolate, or more chopped walnuts.