Combine the flour, sugar, and salt in a bowl. Add the cubed butter and toss to coat.
Dump the mixture out onto a clean surface and use a rolling pin to roll the butter into thin sheets, combining it with the flour. Use a bench scraper to scrape the rolling pin and bring the mixture back into a pile as necessary.
Continue until all of the butter is incorporated into the flour. Mixture will be very flaky.
Return mixture to the bowl and place in the freezer for 10 minutes to chill the butter.
Remove from freezer and add the cold buttermilk. Use a spoon and then your hands to stir the mixture until it comes together into a ball. If mixture is too dry, add additional buttermilk a teaspoon at a time.
Divide the dough in half and flatten into disks. Wrap each disk in plastic wrap and chill in the fridge for at least 1 hour and up to 5 days.
To prepare the pie filling:
In a large skillet add the apple slices, lemon juice, 1 cup of the sugar, the flour and 2 teaspoons of the cinnamon. Set over medium high heat.
Stir frequently as the sugar dissolves and the apples soften. Simmer until apples are soft but still hold their shape and the juices have thickened, about 15 minutes. The whole mixture should appear to be reduced by a third. Set aside to cool while you prepare the crust.
To assemble the pie:
Preheat oven to 400 degrees F.
Let the dough rest at room temperature for five minutes. On a lightly floured surface, use a rolling pin to roll out the dough into a rough 13-inch circle. Dough should be about ¼ inch thick. Transfer dough to 9-inch pie dish. Roll out the second disk of dough.
Dump the cooled apple pie filling into the bottom crust. Transfer the second crust to the top of the pie. Trim edges to about 1 inch.
Fold the edges of the top and bottom crusts together and use your index fingers and thumb to pinch into a pattern. Brush with egg wash. Combine remaining sugar and cinnamon and sprinkle over the top of the pie. Cut a few slits in the crust as vents for steam.
Place pie on a sheet pan to catch any filling that bubbles over. Bake pie until golden on top and filling bubbles through vents, about 1 hour. If crust browns too quickly, cover loosely with foil.
Let the baked pie come to room temperature on a wire rack for several hours before serving, to allow the filling to finish thickening.
Serve with whipped cream or vanilla ice cream, if desired.