Line a 20cm x 20cm baking tin with baking parchment and set aside. Tip the flours, xanthan gum and sugar into a food processor and pulse to combine.

Add the butter and pulse until the mixture reaches a breadcrumb-like texture. Tip out onto your work surface and bring the dough together with your hands. Press into the prepared tin in an even layer and chill for at least 20 mins.

Heat the oven to 180C/160C fan/gas 4. Bake the shortbread for 18-22 mins until lightly golden. Remove from the oven and score along the cooked dough to make 12 rectangles. Leave to cool for 10 mins before cutting through the scored lines completely. Leave to cool completely before serving. Will keep in a biscuit tin for up to a week.