Bring a pot of water to a boil; add the tomatillos, jalapeno, onion, and garlic. Simmer for 10 to 15 minutes until the tomatillos are soft. Drain and cool slightly. Combine the tomatillos, jalapeno, onion, garlic, cilantro, and lime juice in a blender. Add 1/4 cup of water and process to a coarse puree; season with a generous pinch of salt. You should have about 3 cups of this salsa verde.
Combine the shredded chicken and queso fresco in a large mixing bowl. Pour in 1 1/2cups of the salsa verde and fold the ingredients together to moisten. Use as a filling for empanadas. Serve the remaining salsa verde on the side.
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.