Preheat the oven to 400°. Brush four 1-cup ramekins with melted butter. Add 2 tablespoons of the sugar to 1 of the ramekins and rotate it to coat with sugar. Tap the excess sugar into another ramekin and repeat until all the ramekins are coated with sugar.
In a food processor, puree the bananas with the lime juice, egg yolk and 2 tablespoons of the sugar until smooth. Scrape the banana mixture into a large bowl.
In a medium stainless steel bowl, beat the egg whites with a pinch of salt until soft peaks form. Add the remaining 1 tablespoon of sugar and beat until firm and glossy. Using a rubber spatula, beat one-fourth of the beaten whites into the banana mixture, then gently fold in the remaining whites. Spoon the soufflé mixture into the prepared ramekins and tap them lightly on a countertop. Transfer the soufflés to a baking sheet and bake in the center of the oven for about 15 minutes, or until golden brown and risen. Serve at once.
These light yet intensely flavored soufflés taste best with a sweet, smooth Puerto Rican sipping rum.