For the pie crust:

1. Place 3 cups of Cup4Cup flour, sugar, and salt in a food processor and pulse to evenly distribute. Add the cold, cubed butter to the flour and pulse until the mixture resembles a coarse meal. Transfer mixture into a large bowl and set aside.
2. Whisk the egg and the buttermilk together in a small bowl.  Create a well in the flour mixture, gradually stir in the egg mixture with a fork until it forms a ball and gently compress into a dough. Wrap the dough in plastic wrap and rest in refrigerator for 30 minutes. 
3. Cut the dough ball in half. Using extra Cup4Cup flour to prevent sticking, roll one half of the dough into a disk about 12-inches in diameter and 1/8-inches thick.  Lay the rolled dough to into a 9-inch pie dish and trim the edges. If you are preparing a full top crust, roll the other half in the same manner and refrigerate until ready to use. If you are preparing a lattice top, roll out the remaining dough ball to the same thickness and cut into 1-inch-wide strips.

For the pie:

1. Preheat oven to 375°F.
2. In a bowl, combine and mix the peaches, sugars, lemon juice, and vanilla, Cup4Cup flour, salt, ginger, and cinnamon. Let the mixture rest for about 10 minutes at room temperature.
3. Place the peach mixture in the pie dish lined with half of the pie dough.
4. Evenly dot the top of the filling with the 2 tablespoons of cubed butter.
5. Moisten the edges of the pie dough, then lay down the remaining disk of dough or weave the lattice strips on top of the filling. Press the edges to the crust together to seal then use a paring knife to trim the excess dough from the edge. Crimp the crust with your fingers or a fork. If you are preparing a full crust, cut a few slits slit in the top to allow steam to escape.
6. Whisk the egg with the water to make an egg wash. 
7. Brush the top of the dough with the egg wash and sprinkle the coarse sugar on top.
8. Bake on the middle rack for 50-60 minutes or until the filling is bubbling and the crust is golden brown.
9. Cool for at least 1-2 hours before serving.