In a large bowl, sift together the flour, masa harina, baking powder, and salt. Stir in the melted butter. Gradually add 1/2 cup to 3/4 cup of water, working it in with your hands to incorporate; the dough should be easy to handle and not sticky. Form the dough into a ball, wrap it in plastic, and chill for 30 minutes.
Lightly flour your rolling pin and counter. Divide the dough in 1/2 so it will be easier to work with and roll it out to 1/8-inch thickness. Using a 4-inch cookie or biscuit cutter, cut out 10 circles of dough; repeat with the other 1/2.
Spoon 2 generous tablespoons of filling into the center of each pastry circle, leaving a 1/2-inch border. Brush the edges with the egg wash and then fold the dough over in 1/2 to enclose the filling and form a semi-circle. Tightly seal the edges by crimping with the tines of a fork. Chill at least 30 minutes before baking.
Preheat the oven to 375 degrees F. Place the empanadas on a buttered baking sheet and brush the tops with additional egg wash. Using a fork, prick a few holes in the top of the empanadas for steam to escape. Bake for 30 minutes, until the pastry is golden brown.
