Special equipment: a 4-inch cookie or biscuit cutter
For the filling: Coat a large wide pot with extra-virgin olive oil and set over medium-high heat. Add the pork, cumin and some salt, and cook until the pork is cooked through. Using a slotted spoon, remove the meat and set aside. Return the pan to the heat, and coat with more oil if needed. Add the onions, season with salt and cook for 4 to 5 minutes. Toss in the poblanos and cook for 3 to 4 minutes. Add in the garlic and cook for 1 minute. Return the pork to the pan and add the potatoes. Season with salt, and stir to combine. Add the cilantro and stir to combine. Add the wine and scrape the crud off the bottom of the pan. Then add the stock and bay leaf. Taste and season if needed. Bring to a boil and then reduce to a simmer. Partially cover and simmer for 10 minutes. Remove the lid and continue to cook, stirring frequently, until most of the liquid evaporates. The filling should be saucy but not soupy. Remove the bay leaf and cool the filling completely.
For the dough: 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 water, and work 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 refrigerate for 30 minutes.
Lightly dust a rolling pin and a clean surface with flour. Divide the dough in half. Refrigerate half of the dough and roll out the other half of the dough to 1/8-inch thick. Using a 4-inch cookie or biscuit cutter, cut out 10 circles of dough. Repeat with the remaining dough.
In a small bowl, beat the egg with 1 tablespoon water. Spoon 2 generous tablespoons of the 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 to enclose the filling and form a semi-circle. Tightly seal the edges by crimping with the tines of a fork. Chill for at least 30 minutes.
For the sauce: Preheat a grill pan to high heat. Grill the tomatillos on all sides until charred, then remove and cool. When cool enough to handle, coarsely chop the tomatillos.
Coat a large saute pan with oil and set over medium heat. Add the garlic, jalapenos and half of the cilantro, and cook for 2 minutes. Add the tomatillos, season with salt and stir to combine. Cook, stirring occasionally, for 7 to 8 minutes. Remove from the heat and add the lime zest and juice and the remaining cilantro. Transfer to a food processor and puree until smooth.
Preheat the oven to 375 degrees F. Grease a baking sheet.
Place the empanadas on the prepared baking sheet and brush the tops with some egg wash. Using a fork, prick a few holes in the tops of the empanadas for the steam to escape. Bake until golden brown, 30 minutes.
Serve hot with the charred tomatillo salsa.