Preheat the oven to 385 degrees. Line a baking sheet with parchment paper and set it aside.
Combine the peach slices with the sugar, cornstarch, and salt. Mix gently with a rubber spatula. Set this aside.
On a lightly floured surface, roll out the chilled buckwheat pie dough to an 18-inch round circle. Place it carefully on the parchment-lined baking sheet. Remember that this buckwheat pie dough is tender, so be careful when you are rolling and transferring.
Pour the peaches into the center of the pie dough, leaving a 2-inch gap all the way around.
Pleat the edges of the pie dough over the peach filling. Again, the pie crust is tender, so don't get too frustrated as you are pleating the edges.
Combine the egg and milk and mix well. Brush the edges of the pie dough with the egg wash and sprinkle with sugar.
Place in the preheated oven and bake for 25-35 minutes. The peaches should be tender and the crust baked through.
Remove and let it cool for at least 10 minutes. It can be served warm, cool, or even cold. Serve as you like!