Trim the ends off the plantains. Using a sharp knife, cut a slit along the curve of the fruit all the way through the skins. Peel the skins from the plantains, then cut into 5 mm (¼ inch) slices.
Fill a large saucepan one-third full of oil and heat to 180–190°C (350–375°F), or until a cube of bread browns in 15 seconds. Deep-fry the plantains, in batches, for 2–3 minutes until golden. Remove with a slotted spoon and drain on kitchen paper.
Serve sprinkled with chilli flakes and salt flakes.