Combine the flour, sugar (if using), and salt in a bowl. Add the cubed butter and toss to coat.
Use a pastry blender or two knives to cut the butter into the dry ingredients until butter is the size of peas.
Add buttermilk and 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 two 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. Dough can also be frozen (double-wrapped) for up to 2 months.
When ready to roll out, 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 1/4 inch thick. Transfer dough to 9-inch pie dish. Fill pie with desired filling.
Repeat with the second disk of dough. Cut dough and make a lattice over the filling, or leave the circle intact and cover the filling completely, cutting a few vents with a sharp knife.
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 and sprinkle with sanding sugar, if desired.
Bake pie according to the instructions in your specific pie recipe. Different fillings will require different temperatures and time.