Peel the plantains and slice them into 1 cm rounds. Heat the butter to foaming in a heavy skillet over medium heat. Add the plantain slices to the butter and cook for 5 to 8 minutes, or until they are lightly browned and slightly caramelized. Sprinkle the plantain slices with the sugar, allow the sugar to caramelize slightly, then remove the plantains from the skillet. Serve warm.
These plantain slices are also particularly good when served over vanilla ice cream, topped with the caramelized butter from the pan.