In a large bowl, combine the flour and salt. Add the chilled butter and mix until the mixture resembles coarse crumbs. Gradually add the cold water, mixing until a dough forms. Wrap the dough in plastic wrap and refrigerate for at least 30 minutes.

In a skillet over medium heat, cook the ground beef until browned. Drain excess fat. Add the chopped onion and garlic, cooking until the onion is translucent.

Stir in the cumin, paprika, chili powder, black pepper, and salt. Cook for an additional 2-3 minutes. If using, mix in the cilantro. Remove from heat and let the filling cool.

On a floured surface, roll out the chilled dough to about 1/8 inch thick. Cut out circles using a 4-inch round cutter.

Place a tablespoon of the beef filling in the center of each circle. Fold the dough over to create a half-moon shape and press the edges to seal. Use a fork to crimp the edges for a decorative touch.

In a large pot, heat about 2 inches of vegetable oil over medium-high heat. Fry the empanadas in batches until golden brown, about 3-4 minutes per side. Remove and drain on paper towels.

Brush the tops of the empanadas with the beaten egg for a shiny finish, if desired. Serve warm.