Preheat the oven to 200C/400F/Gas 6. Place a baking tray in the oven to heat up.
Grease two ramekins with butter and dust with icing sugar.
In a large bowl whisk the egg whites until they form stiff peaks.
Gradually add the sugar and continue whisking until the mixture is thick enough to leave a ribbon trail when the whisk is lifted.
Carefully add the mashed banana and fold in gently, making sure to maintain volume in the egg whites.
Fill the two ramekins with the mixture. Run a thumb nail or tea spoon handle round the edge to 1cm/½in depth. Place the soufflés onto the hot baking sheet and place in the oven.
Cook for ten minutes, until the soufflé has risen by about an inch.
To serve, place the soufflé on the centre of a plate and dust with icing sugar.