In a shallow, ovenproof pan, caramelise the caster sugar and a little water over a medium heat. Make sure the sugar has melted before the liquid comes to the boil. Remove the sugar from the heat once it has caramelised and then stir in the butter. Slice the bananas and arrange in the pan with the sugar, creating two layers.
Cut a round of puff pastry big enough to cover the bananas and