Line two baking trays with baking paper and set aside while you make the dough.

Add the butter and sugars to a mixing bowl and beat with an electric whisk for 1-2 minutes until pale. Sift in the flour and xanthan gum and, using a wooden spoon, start to bring the mixture together into a dough. It may look quite dry and crumbly at first but it will start to come together - be patient.

Once the mixture begins to look more dough-like, add the cranberries, chocolate chunks and pistachio pieces. Use your hands to bring the mixture together into a smooth ball of dough, with the pieces evenly distributed.

Roll the dough out to around 1cm thick and cut out 20 cookies, placing them on the baking sheets with 1-2cm between them. You may need to re-roll the dough a couple of times. Place the baking trays with cookies on them in the fridge and chill for at least 30 minutes.

Preheat the oven to 180'C/ Gas 6 / 160'C Fan. Once chilled, bake the shortbread biscuits for 25-30 minutes until slightly golden on top. Remove from the oven and cool on the baking trays for 15-20 minutes until they harden, before removing onto a wire rack to cool completely.

To decorate, break the white chocolate into pieces into a bowl over hot water and melt. Once melted, dip half of the shortbread biscuits into the chocolate, lay on a sheet of baking paper and sprinkle with pistachio pieces. Alternatively, you could drizzle the chocolate on top, or do half-and-half. Chill in the fridge until set.