Peel the plantains by cutting off the ends and making a lengthwise incision along the ridges. Carefully slide your thumb under the skin and peel it away.
Cut the plantains into thick slices, about 1 inch (2.5 cm) thick.
Heat vegetable oil in a deep frying pan or skillet over medium-high heat. The oil should be about 1/2 inch (1.25 cm) deep in the pan.
Don’t wait for the oil to heat to place the plantains. This will make sure the plantains cook all the way through.
Add the plantain slices to the pan, making sure not to overcrowd them. Fry them in batches, if necessary, at medium heat.
Cook the plantain slices for about 2-3 minutes on each side or until they turn golden brown. Use a tooth stick to check if the plantains are cooked. Use a slotted spoon or tongs to flip them over.
Remove the fried plantains from the oil and place them on a paper towel-lined plate or tray to drain excess oil.
While they are still hot, use a glass plate with a flat bottom to press the tostones to flatten each slice into a round disc. Do it gently so the tostones don’t fall apart.
Then with a roller, gently keep flattening the plantain slice so it things out in a circular shape.
Place the fattened plantain on a muffin tray and shape it around the circular shape. Don’t worry if there are loose parts. Just make sure there are no holes in the cups.
Place the tray on the fridge with the plantain cups for at least 10 minutes before frying the second time.