Combine the masa harina, water and salt in a large mixing bowl and stir until smooth. The dough should be slightly sticky and form a ball when pressed together. To test, flatten a small ball of dough between your palms. If the edges crack, add water to the dough, a tablespoon at a time, until a test piece does not crack. Cover with a damp towel until ready to use.
Heat the oil in a large skillet over moderate heat. Add the onion and saute until soft, 3 to 5 minutes. Ad the garlic, jalapeno, cumin, salt and pepper and saute 1 minute longer to release the aromas. Add the tomato, potatoes and corn and cook, stirring occasionally, until the liquid from the tomatoes is absorbed, 2 to 3 minutes. Remove from the heat, stir in the parsley and cheese and correct the seasoning. Set aside to cool.
Divide the dough into 24 golfball size pieces, moistening your hands with water to prevent sticking if necessary. Flatten balls to 3 inch disks. Place a heaping teaspoon of stuffing on the empanada and close by pressing the edges together with your fingers. Shape into a half moon. Repeat until all the half moons are formed. Preheat the oven to 375 degrees F. Line a baking sheet with parchment paper and arrange the empanaditas on top. Brush with the glaze and prick with a fork to make steam holes. Bake for about 2530 minutes, or until golden. Remove and transfer to a rack to cool slightly. Serve warm.
c.1997, M.S. Milliken & S. Feniger, all rights reserved