On a clean flat surface or in a large shallow bowl, combine the flour, sugar, cornstarch and salt. Add the butter to the dry ingredients and, using your tool of choice, cut the fat into the flour with speed and patience, until the fat has been reduced to small pea-sized chunks. Avoid using your fingers because the heat from your hands will melt the fat and encourage gluten development.
Gently spread out the flour mixture to expose as much surface area as possible. Drizzle in about half of the milk mixture and toss to combine. Drizzle in the remaining half and work just until the dough comes together. There should still be visible chunks of butter in the dough. If the dough seems too dry add one tablespoon of milk at a time until the desired consistency is reached.
Divide the dough evenly and wrap each half tightly in plastic wrap. Let the dough rest in the refrigerator for at least 1 hour. Reserve the second dough round for another use for up to a week in the refrigerator or in the freezer for up to 2 months.
Preheat the oven to 425 degrees F.
Roll out the dough into a circle about 11 inches in diameter. Transfer it to a 9-inch pie plate, trim the overhang to about 1 inch, tuck the overhang under and crimp decoratively. Fit a piece of aluminum foil to cover the inside of the crust completely. Fill the crust up to the edges with pie weights or dried beans and place it in the oven. Bake the crust for 10 minutes remove the foil and pie weights and bake until the crust is dried out and beginning to color, about 10 minutes. Set aside to cool. Lower the oven to 350 degrees F.
Brush the edges of the pie crust with the egg wash.
For the candied mint leaves: Pour the sugar onto a plate or shallow bowl. Whisk the egg white in a small bowl until frothy. Using a pastry brush, paint the leaves with the egg white and dredge in the sugar, tapping off any excess. Allow them to harden on a wire rack at room temperature for at least 3 hours.
For the simple syrup: Stir together the sugar and 1 cup water in a saucepan over high heat. Bring the mixture to a boil, then remove it from the heat. Stir in the mint leaves and allow them to steep for at least 30 minutes. Strain and discard the mint. The syrup can be refrigerated for up to one week. To store it longer, add 1 teaspoon of vodka.
Place the crust on a baking sheet and pour in the filling. Bake until the filling has just set and is still slightly wobbly in the center, 20 to 25 minutes. Let cool completely.
For the topping: Whip the cream with 2 tablespoons of the mint simple syrup using a whisk, stand mixer or hand mixer until stiff peaks form. Pile the whipped cream on top of the pie and refrigerate at least 30 minutes.
To serve, top the pie with the candied mint leaves.