In a large bowl combine the flour, cornmeal, and salt. Slice the butter into small ¼-inch cubes. Work these into the flour using a pastry blender until the mixture is coarse.
Add ice water to the mixture, a ¼ cup to start and then 1 tablespoon at a time, using a spoon to toss the mixture together after each addition: add water only until the dough is moist enough to hold together when you squeeze some in your hand.
Turn the dough out onto a lightly floured surface, and knead gently 4-5 times. Divide the dough in half, flatten each half slightly with the palm of your hand and wrap the disks in plastic wrap. Chill the dough disks in the fridge for 30 minutes or until firm.
Make the filling:
Slice the potato into small cubes, and place them in a saucepan with just enough cold water to cover them. Bring the water to a boil, reduce the heat and cook the potatoes for about 5 minutes until they are just tender. Drain the potatoes and set them aside.
Place a large skillet over medium-high heat. Crumble the ground beef and chorizo into the pan, and add in the diced onion and garlic. Cook the mixture for about 7 to 10 minutes until the beef and chorizo are cooked through. Drain off the grease, then return the pan to the stove. Add in ½ cup of water, the cooked potato, salt, pepper, and chili powder. Stir everything together and cook for another 3 minutes. Remove the pan from heat and set the mixture aside to cool.
Assemble the pies:
Have some water ready at your work area, as well as a large parchment paper-lined baking sheet.
Unwrap one dough disk onto a lightly floured surface and roll out to about ⅛-inch thickness. Cut 5-inch circles out of the dough; gather and re-roll the scraps to get additional circles.
Spoon about 3 tablespoons of filling onto a dough circle. Dip your finger with water and moisten the entire edge of the circle. Bring the far edge of the circle towards you to meet the other edge, press the edges together firmly. Gently pull a corner and fold it in, then pull the corner created by that fold and fold it in. Repeat this along the edge of the empanada until it's sealed all around.
Place the empanada on the prepared baking sheet. Repeat with the remaining dough circles and filling, also using the second disk of dough from the fridge. Cover the empanadas with plastic wrap and chill them for 30 minutes.
Preheat the oven to 375° F.
Cut vents into the tops of the chilled empanadas. Beat together the egg and ¼ cup of water. Brush this wash over the empanadas using a pastry brush. Sprinkle a little Maldon sea salt on the tops.
Bake the empanadas for about 30 minutes, until they are lightly browned on top. Remove them from oven and serve while warm. Leftover empanadas should be stored wrapped in the refrigerator for up to 3 days.