* Be sure not to over flour the dough when rolling, a more pliable and wet dough will yield fewer cracks. Also, be sure to keep the dough as cold as possible. If the dough becomes soft while handling put it in the fridge or freezer for 5 to 10 minutes.
Place all dry ingredients into a food processor and pulse evenly to distribute. (Or if you do not have a food processor, place dry ingredients into a large bowl.) Add the butter to the flour mixture, pulsing. Pulse until the mixture resembles coarse meal. Place mixture into a large bowl and set aside. Combine the egg and the buttermilk together in a small bowl. Create a well in the flour mixture, gradually whisk in the egg mixture with a fork. Mix the dough until it forms a ball. All dough to rest in the fridge for 30 minutes.
Preheat oven to 350 degrees. Place the potato on a sheet tray and bake in the oven until tender and fully cooked. Let it cool slightly and remove the skin. Place into a medium bowl and mash. In the meantime, preheat a small sauté pan over medium-high heat. Add in canola oil and onions and sauté until transparent. Add in minced garlic and sauté 2 minutes. Add this to the bowl with the mashed potato.
Add to the bowl: sour cream, cheese, cayenne, and parsley. Mix until well combined and season to taste with salt and pepper. Store mixture in the fridge until ready to use.
Cut the rested dough in half. Using extra Cup4Cup flour for dusting a flat surface, roll out 1 portion into a circle about 1/8 inch thick. Keep other half of the dough refrigerated until ready to use. Using a round cutter about 3 inches in diameter, cut out circles from the dough. Place circles onto a sheet tray lined with parchment. Place about 1 teaspoon of filling in the center of each circle. Fold over 1 side to create a half-[circle. Be sure that no air bubbles remain in the center. Crimp the edges together with a fork. Place finished empanadas onto a tray about 1 inch apart. Continue rolling, cutting, and filing the dough until no dough remains. Brush the empanadas with the egg wash and bake in the oven for 15-20 minutes until golden brown. Serve hot.