Beat the butter, then cream it with the sugar and vanilla extract until light and fluffy. Hard margarine can be used instead of butter to make them dairy free and vegan. It will just change the taste and mouthfeel slightly.
Stir in the flour, baking powder and xanthan gum and mix together to form a stiff dough. It should be short – so prone to falling apart, but still hold together. The xanthan gum should prevent the biscuits from falling apart without any gluten.
Mix in the chocolate chips until they are evenly spread out through the dough. Any kind of chocolate chip will work fine, but I personally prefer dark chocolate. If you do not have chocolate chips, the shortbread will be fine without them or you can use chopped up chocolate (or any leftover chocolate).
Divide the mixture into 10 balls, arrange on a baking sheet. Make sure to leave space between each biscuit, as they can spread. Squish each ball down, so that they are mostly flat so that they will bake evenly.
Chill for 15 mins, then bake for 12-15 mins until light golden. Make sure to leave them to cool before eating, otherwise they will fall apart.
The Whistler 