Peel the plantains by cutting off the tip on each end. Make three cuts in each plantain by running the tip of a sharp knife down the length of the plantain, cutting just through the peel. Carefully remove the peel.
Cut the plantains into 1-inch thick rounds.
Place the water in a large bowl and stir in the salt just until dissolved. Add the sliced plantains and let soak for at least 10 minutes and up to 30 minutes. Drain well and pat dry.
Pour enough oil into a large skillet to cover the bottom by ½-inch. Heat over medium-high heat until warmed but not smoking. Working in batches if needed, add a single layer of plantains to the oil. (The oil should bubble softly not vigorously and be around 325F to 335F. Adjust the heat if needed.)
Fry for about 4 minutes per side or until lightly golden. Transfer to a paper-towel-lined plate to drain. Remove the oil from the heat. (At this point, the plantains can be held for a few hours and finished when ready to serve.)
Place a piece of plantain between two layers of plastic wrap or parchment paper. Using the bottom of a glass or a can, gently flatten the plantain. Repeat with the rest of the fried plantains.
Return the skillet to medium-high heat, adding more oil if needed. Fry each flattened plantain until golden and crispy, about 3 minutes per side. Transfer to a paper-towel-lined plate to drain and immediately sprinkle with salt, if desired.
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Notes
I like using sea salt flakes to top the tostones before serving as the flaky salts add an extra crunchy texture.
While similar in shape, you cannot swap plantains for bananas. The high starch content keeps the plantains from falling apart as they cook.
Avoid overcrowding the skillet with the plantains, as doing so will prevent them from cooking unevenly.
If you have a tostonera, it is perfect for flattening the tostones. Other methods of flattening the tostones are by pressing down with a plate, a sheet pan, or even a spoon!
If you do not have a thermometer, rip off a tiny amount of the plantain and add it to the oil. If it bubbles, then the oil is ready.