Try grinding the walnuts to make a flour - it gives a different texture. This bread is also good for toasting.
You will need a 900g loaf tin and ...
1 Set your oven to 170°C/Gas3.
2 Beat the butter and sugar together in an electric mixer, until they are well blended. Beat in the eggs, one at a time, then add the vanilla and bananas and mix well, but not too much. It will appear to be curdled, but it is OK!
3 Sieve the flour, xanthan gum and baking powder together into a smaller bowl. Add to the banana mixture and mix slowly until all the dry ingredients are evenly blended. Remove the bowl from the mixer and with a large spoon or spatula stir in the walnuts.
4 Line the base of a loaf tin with lightly oiled baking parchment. Spoon the mixture into the loaf tin, tamper down (bang the tin on the work surface) and bake in the centre of the oven for about 70 minutes or until a skewer or cocktail stick comes clean out of centre.
5 Take care not to under bake.
6 Cool in the tin on a wire rack for about 10 minutes then turn out on to the wire rack and allow to cool properly.