2) Put the bananas, brown sugar, lemon juice and rum in a medium saucepan. Cook uncovered, over a medium heat, stirring occasionally, until the bananas begin to soften, about 7 minutes. Mash the bananas with a potato masher or large fork until mostly smooth; continue cooking to make a thick puree, about 8 more minutes. Cool to room temperature.
3) Position the oven rack in the lower third of the oven and preheat the oven to 220C/Gas 7.
4) Whip the egg whites and salt in a large bowl with an electric mixer at medium-high speed until foamy. Increase the speed to high, gradually pour in the 2 tbsp sugar, and whip until the whites hold soft peaks. Fold a quarter of the whites into the banana mixture and then fold in the remaining whites. (Don't dally here; timing is everything with souffles. Whip, fold, and get the souffle in the oven without missing a beat.)
5) Evenly divide the batter among the prepared ramekins. Bake the souffles until well puffed and golden, about 15 minutes. Dust with a light sprinkling of cocoa powder, if desired. Serve immediately.