Preheat your oven to 180°C / 350°F. Lightly grease a 23×13cm (9×5 inch) loaf tin.
Mix the dry ingredients. In a large bowl, combine the gluten free flour, baking powder, and xanthan gum. (Keep the sugar separate for now.)
Cream butter and sugar. In a separate bowl, cream together the softened butter and brown sugar until light and fluffy.
Add the eggs. Beat in the eggs one at a time, continuing to mix until the batter is light and airy.
Stir in bananas and vanilla. Mash the ripe bananas and add them to the wet mixture along with the vanilla extract. Stir to combine.
Bring it together. Fold the banana mixture into the flour mixture, stirring just until moistened — don't over-mix.
Pour and bake. Pour the batter into your prepared loaf tin and bake for 50–60 minutes, or until a knife or skewer inserted into the centre comes out clean.
Cool before slicing. Leave the loaf to cool in the tin for 10 minutes, then turn out onto a wire cooling rack.
