To form the dough, cut the plantains into 3 or 4 inch pieces, without peeling. Boil the plantains in salty water for about 20 minutes, then peel them. Mash with a potato ricer or a food mill or just really hammer on them -- you want no lumps. Mix with the masa harina until you can knead the dough without it being too sticky. You might need more or less than 1 cup, so start with 1/2 cup and go from there. Put the dough in a plastic bag for at least 20 minutes to hydrate.
Heat the olive oil in a pan over medium-high heat, then sauté the onions and serranos until soft. Add the garlic and the tomatoes and cook another minute or two, then add the olives and the shrimp, and cook, stirring often, until the shrimp are cooked. Chop the shrimp small. Add the cilantro, taste for salt and pepper, and let this cool completely while the dough is hydrating.
Divide the dough into golf ball-sized balls. You will get anywhere from 8 to 12 depending on exact size you made them and the size of the plantains. Keep the balls in the plastic bag as you work with them so they stay moist.
Get several cups of vegetable oil hot in a large, wide pan. You want it to reach about 325°F. Set a baking sheet in your oven and put a cooling rack on top of it. Set your oven to 200°F.
Using your tortilla press (lined with two sheets cut from a plastic bag), press out a "tortilla" about 1/8 inch thick, so thicker than an actual tortilla, but still pretty thin. Remove the top plastic sheet and spoon about a tablespoon or two of the filling on one half of the dough circle. If you don't have a tortilla press.If you don't have one, roll the balls of dough out between two sheets of wax paper or plastic wrap until you have rounds of about 1/8-inch.
Fold over the other half and gently press it to seal. Carefully lift the empanada off the plastic -- it's fragile right now -- and use the tines of a fork to secure the seal. Make a couple of these, then start frying in batches of 2 or 3, making more as you fry.
Fry the empanadas for a couple minutes per side. They will brown in a hurry, so keep an eye on them. I find 2 minutes per side works. Put the finished empanadas on the rack in the oven while you do the rest.
Notes
These empanadas are best right out of the fryer, but they're still good at room temperature. If for some reason you have leftovers, they are best reheated in a toaster oven or by re-frying for a minute or two.
Keys to Success
Make sure the oil is hot, but not too hot or the dough can burn. It's susceptible to that. 325°F is the sweet spot.
If you have leftover filling, it's good on tortillas or in a burrito, or with rice.
As I mention, crab, crawfish tails or chopped lobster are all good alternatives.
Black or green olives are equally good.
If you hate spicy food, use an Anaheim, poblano or green pepper instead of the serranos.