Sift the flour into a bowl and stir in the sugar. Whisk together the egg and milk in a jug, then pour into the flour mixture and whisk until smooth. Melt 10g butter, then stir into the batter. Leave to rest for 10 minutes.
Smear a little of the oil in a small frying pan over a medium heat. Pour in a quarter of the batter. Cook for 1 minute, then flip and cook on the other side for 30 seconds. Transfer to a warm plate. Repeat to make 4 pancakes.
Put the remaining butter, brown sugar and juice in a pan and heat gently, stirring, until bubbling. Add the banana and warm through for a few minutes. To serve, top the pancakes with the bananas, then spoon over the sauce and the crème fraîche.