Cut off the ends of the plantains and then cut each plantain into 1 inch pieces. Add the cut plantain pieces into a large pot of water and bring to a boil under medium high heat.
Cook the plantains until tender, about 10 to 12 minutes or longer as needed. If you were to poke through one of the plantain pieces with a knife or skewer, it should go through easily without resistance.
Drain the cooked plantains and set aside to cool for 10 to 15 minutes.
As the plantains cool, make the beef filling.
Heat some oil, about 1/2 tablespoon, in a saucepan over medium heat. Add in the onions and jalapeño peppers. Cook mixture while stirring until soften, about 3 to 4 minutes.
Next, add in the ground beef and use a large spoon to break up the meat. Mix in the sofrito, tomato paste, Sazon and adobo seasoning. Cook ground beef mixture until the meat is fully cooked and no longer pink, about 6 to 8 minutes or longer as needed. (If meat mixture appears dry as it cooks, add a few tablespoons of water and continue to cook until the beef is fully cooked through.)
Remove the pan from the heat and set aside to cool the beef filling.
Into a large mixing bowl, peel and add the cooled plantains. (Plantains should not be wet. If plantains appear wet, pat dry with paper towels to remove excess moisture.)
Use a potato masher to mash the plantains into a smooth mixture. Next, add in the cornstarch (starting with 5 tablespoons) and the seasonings (paprika, adobo, garlic and onion powders).
Mix ingredients together until a smooth, soft dough forms. The dough should be able to hold its shape and not be overly sticky. If the dough is too soft, mix in additional cornstarch, 1 tablespoon at a time until a soft but not too sticky dough forms. You should be able to scoop and form the dough into a ball.
Line a large baking sheet with parchment paper. Set aside. Divide the plantain dough into (4) equal pieces and shape into balls.
On a work surface, place down a sheet of parchment paper, add one of dough balls into the center and top with another sheet of parchment paper.
Using a rolling pin or hands, flatten the dough into a 6 inch circle and remove the top parchment paper.
On the top half of the dough, add 1 to 2 tablespoons of cheese. Spoon over the cheese a portion of the cooled beef filling and top with an additional 1 to 2 tablespoons of cheese.
Use the bottom parchment paper to fold over the lower half of the dough over the filling. Press the edges of the plantain dough together to seal the empanada and transfer to the prepared baking sheet. Repeat process with the remaining ingredients for a total of (4) empanadas.
Heat oil (about 1 1/2 to 2 cups) in a large frying pan or skillet over medium heat to 350F.
In batches of (2), fry the empanadas, about 1 to 2 minutes per side, or until golden brown. Transfer to a large plate or baking sheet that is lined with paper towels to remove any excess oil.
Serve plantain empanadas immediately while still hot as is or with your preferred sauce. I serve empanadas with a Cilantro Lime Sauce (link to recipe is shared in the notes section below).