Cube the butter and then add to the freezer for 15 minutes.

Add the flour, salt, and cold butter to your food processor and pulse until it resembles coarse sand. 

Take the cold water, add about 1/3 of it to the mixture, and pulse. Repeat again with another 1/3 of the water, and then again with the remainder. At this point, the pie crust should pull away from the sides and start to form a ball. If it still hasn't done this, add 1 more tablespoon of water at a time until it pulls away from the sides.

Remove the dough from the food processor and push it together fully using your hands. Divide the dough into two equal parts and form it into a small disk shape. Cover the dough with plastic wrap or parchment paper and refrigerate for at least one hour.

Combine cherries and sugar in a large saucepan and heat over medium heat. Cook until the cherries start to release their juices.  

In a small bowl, combine the cornstarch and 2 tablespoons of water and stir until all the clumps in the cornstarch are out. Add to the cherry mixture and stir until fully combined. Keep over the heat until the cherries start to thicken. 

Remove from the heat and add in the vanilla extract. Let the cherry filling cool while the dough is chilling in the fridge. 

Preheat the oven to 400 degrees.

Remove the dough from the fridge and let it sit for a couple of minutes so it will soften a little bit to make it easier to roll out

Add the dough to a lightly floured surface, and roll out using a rolling pin that has also been lightly coated in flour. Roll the dough out into a circle until it is about 12 inches in diameter. You want the dough to be bigger than the pie pan. 

Using your rolling pin, lift the dough up and wrap it around the rolling pin and then transfer it to the pie pan, pressing it to the bottom and the sides of the pan. 

Pour the sour cherry pie filling into the pie pan and spread it evenly with a spatula. 

Repeat rolling the second dough out and add to the top of the pie for the top crust.

Cut the excess pie dough off on the outside of the crust. Then, tuck the edges of the crust underneath itself to seal it up, and then flute the edges. 

Add one egg and one tablespoon of water to a small bowl and whisk to combine. Brush the egg wash on top of the pie crust. Add 4 cuts to the pie crust on top for vents to allow steam to exit the pie while baking.

Add the pie to the preheated oven on the middle rack and bake for 35-45 minutes, or until the crust is golden brown and the cherry filling is bubbling. Check the pie halfway through and if the edges are becoming too brown you can tent the edges with foil. 

Let the pie cool for at least 30 minutes before cutting into it. The longer the pie cools, the thicker the filling will be.