Put the golden syrup, orange zest, sugar, butter, ginger and cinnamon into a large pan over a medium heat for 5 mins, stirring occasionally, until the butter has melted and the sugar has dissolved, taking care not to bring the mixture to a rolling boil. Remove from the heat, add the bicarbonate of soda and stir to combine. Leave to cool for 5 mins, then mix in the egg before sifting over the flour and xanthan gum. Beat with a wooden spoon until you have a smooth, glossy dough. It will be quite soft at this point, so leave to firm up in the pan for 10 mins, before shaping into a disc, wrapping in baking paper and chilling in the fridge for 1 hr or until firm enough to roll.