For the pastry: Combine flours, confectioner's sugar and butter in a food processor. Pulse until mixture resembles breadcrumbs.
Add 2 tbsp water and pulse until a dough forms around the blades; add more water as needed to bring the dough together.
Turn out dough when ready and knead very briefly before shaping into a disc. Wrap in plastic wrap and chill for 30 minutes.
After chilling, preheat oven to 180°C (160° fan) | 350F | gas 4. Roll out pastry on a lightly floured surface into a round approximately 0.25" thick.
Use pastry to line a 9" fluted tart tin, pressing it into the base and sides. Cut away any excess, overhanging pastry and prick the base all over with a fork.
Line pastry with a sheet of parchment paper andn fill it with baking beans. Blind-bake for 12-15 minutes until golden at the edges.
Discard beans and paper and return pastry to the oven for a further 4-6 minutes to brown the base. Remove again and brush the tart with the beaten egg before leaving to cool on a cooling rack.
For the filling: Reduce oven to 170°C (150° fan) | 325F | gas 3.
Beat together all ingredients for the filling in a large mixing bowl until smooth and creamy. Pour into the baked base.
Bake for 35-45 minutes until filling is set and a knife inserted towards edge of the filling comes out clean.
Remove from oven and let cool to room temperature. Wrap in plastic wrap and chill for at least 4 hours, preferably overnight.
To serve: Whip cream in a large mixing bowl until softly peaked. Cut pie into slices and serve with whipped cream on top.