In a large mixing bowl or food processor, combine flour, cornstarch, sugar and salt.
Add in cold butter and mix until the butter is in pea sized chunks.
Add cold milk and mix until just combined.
Roll the pie dough into a medium sized square. Cut in half and stack the halves on top.
Repeat once, then roll the dough into a flat round.
Wrap in plastic wrap and chill for at least one hour to let the gluten in the dough relax.
On a lightly floured surface, with a rolling pin, roll out the dough into a 12-inch (diameter) circle.
Transfer the dough to a 9-inch pie plate and press into the pie pan. Shape the edges.
Chill again and preheat the oven to 375℉.
While the crust is chilling, make the filling.
In a large bowl, combine all the filling ingredients and mix until combined.
Make the shortbread topping. In a small bowl, combine butter, sugar and vanilla.
Add flour and salt and mix until just combined.
Take the crust out of the fridge and fill with the filling.
Sprinkle an even layer of the shortbread topping on top of the pie.
Using a pastry brush, brush the pie crust edges with egg wash (egg mixed with milk), milk, or cream.
Bake pie for 1 hour and 25 minutes, or until the edges and topping are a deep golden brown. The center should be bubbling or at least darkened in colour. The filling may look liquid, but it will thicken as it cools.
Cool for at least 3 hours or until the pie is fully at room temperature before slicing.