Beat together the soft butter, sugar, salt and orange zest until light and fluffy. Sift the flour, baking powder, mixed spice, cinnamon and xanthan gum into a bowl. Add a quarter of the flour mixture and an egg to the butter mixture and beat until smooth again. Repeat, adding another quarter of the flour and an egg and beating again, until they are all added. Finally add in the treacle and dried fruit, gently folding until combined.