Preheat the oven to 180°C/Fan 160°C.
Grease a 2lb loaf tin and line with parchment paper.
Beat the caster sugar, 75g soft light brown sugar and the butter in a bowl until light and pale (using an electric mixer will speed this up).
Add in a little gluten free flour and one egg. Mix through, repeat with the other egg and the rest of flour, baking powder, vanilla and a pinch of salt until everything is mixed through and smooth.
Peel the bananas and mash two of them well.
Add to your cake mixture and stir through along with the chocolate chips.
Pour the mix into the prepared loaf tin, slice the remaining banana in half lengthways and arrange on the top of the loaf.
Sprinkle with the remaining brown sugar then place in the oven for approximately 50-55 minutes.
Cool for 10 minutes in the tin before turning out onto a wire rack to cool completely.