To make the sauce, place the peanut butter, lime rind and juice and measurement water in a saucepan and cook over a low heat, stirring, until smooth.
Place the flour and sesame seeds in a bowl and make a well in the centre. Add the egg yolk and milk and gradually whisk into the flour to form a thick batter. Whisk the egg white in a clean bowl with a hand-held electric whisk until stiff, then fold into the batter. Tip the bananas into the batter and stir lightly to coat.
Heat about 5 cm (2 inches) of oil in a wok or large frying pan until a drop of batter rises to the surface surrounded by bubbles and starts to brown. Add the bananas and fry for 3–4 minutes until crisp. Remove with a slotted spoon and drain on kitchen paper.