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 half of the flour mixture and an egg to the butter mixture and beat until smooth again. Repeat, adding the other half of the flour and the final egg and beating again until combined. Finally add in the treacle and dried fruit, gently folding until combined.