Put the flours in a large bowl (add a pinch of salt if you're using unsalted butter) and mix with a fork.
Put the cold cubes of butter in the flour mixture and with your thumbs push the butter to flatten it into the flour. Now add the cold water, about 5 tbsps at first and push the flour against the side to the bowl (adding additional cold water tbsp by tbsp as needed) to gather into a dough ball.
Place a parchment larger than your pie plate, on the counter and gently roll out the pie dough into a disc larger than the diameter of a 8-inch pie plate (you need enough pastry overhanging to be able to crimp its edges.
Now place the pie plate upside down on the rolled-out pastry, and holding both ends of the parchment along with the rim of the pie dish, gently flip so that the pastry sits in the dish. Using your fingertips, ease the pastry into the plate and remove the parchment. Crimp or pinch the edges with your fingertips to create a pretty border. Trim any extra pastry.
Heat the oven to 180°C. Press a sheet of aluminium foil on the unbaked pastry shell, tucking the edges into the sides. Fill the foil with enough sugar to weigh down the pastry shell. Bake for 10 min, remove from the oven, lift out the foil with sugar and cool the shell.
While the pie shell is cooling, toss the sliced apples with the lemon juice, brown sugar, cinnamon powder and cornflour. Let the apples macerate for 5-7 minutes.
Arrange the apples in the cooled pie crust and spoon some of the residual liquid on the filling (you can drizzle a little more when the pie is almost done). Beat together the egg yolk and milk and brush the crust with it using a pastry brush. Bake at 190°C for 35-40 mins or till the apples are cooked with just enough glaze-like liquid remaining to keep the pie moist. Remove the pie from the oven, cool for at least 20 minutes before serving.