To prepare dough: Combine flour, cornmeal, sugar, baking powder and salt in a medium bowl. Add oil and work with your fingertips until the mixture becomes crumbly. Add water and knead until dough just comes together (do not overwork). Press the dough into two disks. Wrap in plastic and refrigerate for at least 30 minutes or overnight.
To prepare filling: Heat a medium skillet over medium-low heat. Add pumpkin seeds and cook, stirring, until lightly browned and fragrant, 2 to 3 minutes. Transfer to a blender or spice mill and grind to a fine powder.
Cook ground beef in a large nonstick skillet over medium heat, crumbling with a wooden spoon, until no longer pink, about 4 minutes. Drain on paper towels. Add onion to the pan and cook, stirring often, until softened, 2 to 3 minutes. Add bell pepper and garlic; cook, stirring often, until fragrant, about 1 minute. Stir in tomato, cumin, broth, vinegar, potatoes, salt, ground pumpkin seeds and the beef. Reduce the heat to medium-low; cover and cook for 10 to 15 minutes to intensify flavors and reduce liquid. Fold in scallions and hard-boiled egg. Let cool completely in the refrigerator.
Meanwhile, prepare Aji Sauce. Set aside.
To assemble empanadas: Preheat oven to 350 degrees F. Coat 2 baking sheets with cooking spray. On a well-floured surface, roll out one disk at a time to about 1/8 inch thick. Cut circles of dough using a 4 1/2-inch round cookie cutter (see Tips). Make 24 circles altogether.
Brush egg glaze around the edges of the dough circles. Place about 2 tablespoons filling in the center of each circle. Fold filled circles in half, press edges together and crimp with a fork. Place on the prepared baking sheet. Brush egg glaze over the empanadas.
Bake the empanadas until golden and crisp, switching the position of the pans midway, 55 to 65 minutes. Let cool for 5 minutes. Serve warm, accompanied by Aji Sauce.
You can use 1 cup whole-wheat pastry flour to replace 1 cup all-purpose flour.If you don't have the right-sized cookie cutter, an empty 28-ounce can or plastic lid will also work.Ingredient notes:Hulled pumpkin seeds, also know as pepitas, are medium-dark green and have a delicate nutty flavor. They are sold salted, roasted and raw, and can be found in the health-food sections of many supermarkets. Sherry vinegar, made from the popular Spanish fortified wine, has a subtly nutty flavor. It is great in salad dressings and can be found in most grocery stores.