Preheat oven to 180C. Grease and line a 11cm x 21cm loaf tin.
Combine flours, Natvia, salt, xanthan gum and cinnamon in a large bowl. Set aside.
Whisk together the butter, eggs and vanilla. Pour the wet ingredients into the flour mixture.
Add the banana and mix until combined. Pour into the prepared tin and bake for 45-50 minutes or until a skewer is inserted comes out clean. Leave in the tin to cool for 10 minutes before turning out onto a wire rack to cool.
To make the cinnamon butter, whisk the butter ingredients together until creamed and pale. Serve on warm banana bread.