For the dough, cut the butter into small chunks and chill in the freezer for around 15 minutes.
In a bowl of a food processor, pulse flour, sugar, and salt a few times. Add in COLD cubed butter, and pulse until butter is cut into the flour, but still has visible chunks no larger than a pea. You can also use two forks or a pastry cutter. (You can do it by hands, using your fingers, but after you add in the butter, put the bowl in a freezer for 15 minutes. The warmth from your hands will soften the butter.)
While pulsing on and off, slowly add in the buttermilk and continue to pulse slowly until it’s all combined. The dough will be fairly crumbly and will barely hold together, but when squeezed together it should hold its shape. If it seems to need it, however, add in more buttermilk a teaspoon at a time.
Lay a sheet of plastic wrap on the counter. Split the dough in half and dump the half of it on a prepared plastic wrap. Do the same with the rest of the dough. Gather the edges and press tightly together to form a disk, wrap in the plastic wrap, and refrigerate for at least 2 hours.
Place oven rack in the bottom third of the oven. Preheat oven to 200°C (400°F).
When the dough is completely chilled, take it out of the fridge to rest at room temperature for about 10 minutes. This makes it easier to roll out.
For a 23 cm(9-inch) pie dish, roll one half of the dough on a lightly floured surface into a rough 30 cm (12-inch) circle. Roll the dough onto your rolling pin and gently place it into the pie dish. Brush the base and sides of the pie dough with the beaten egg white, and put the prepped pie shell in the fridge for at least 15 minutes. Roll out the second dough half, cut into strips or into any shape you want, transfer to a cookie sheet, and put it into the fridge to chill, while you make the filling.
To make the filling, in a large bowl stir together strawberries, sugar, cornstarch, lemon juice, and vanilla extract. Let stand for 15 minutes.
To assemble the pie, take the pie dish out of the fridge, and using a slotted spoon, spoon filling into the pie crust, leaving some of the liquid in the bowl.
Apply a lattice top
, crimp or flute the edges to seal, and place your completed pie in the freezer for 10 minutes.
Whisk together egg and 1 tablespoon water until smooth; brush onto crust. Sprinkle the top with sugar, if using. Place pie on a parchment-lined baking sheet. Bake for 15 minutes. Reduce oven temperature to 180°C (350°F); bake for 40-45minutes more (bake the pie until the filling juices are visibly bubbling), covering with foil to prevent excess browning, if necessary. Remove from oven and let cool to room temperature for at least 2 hours. Store in the fridge before serving. Serve warm or cold.