Line 2 baking sheets with nonstick baking paper.
Place the flour, xanthan gum, baking powder and butter in a food processor and whizz until the mixture resembles fine breadcrumbs. Alternatively, mix together the flour, xanthan gum and baking powder in a large bowl. Add the butter and rub in with the fingertips until the mixture resembles fine breadcrumbs. Stir in the sugar and orange rind.
Add the egg and golden syrup and combine to form a firm dough. Turn the dough out on a surface lightly dusted with rice flour, roll out to a thickness of 5 mm (¼ inch) and use animal cutters to cut out 20 biscuits, rerolling the trimmings as necessary. Transfer the biscuits to the prepared baking sheets.
Place in a preheated oven, 160°C (325°F), Gas Mark 3, for about 10 minutes until golden. Remove from the oven, leave for a few minutes on the baking sheets to harden, then transfer to a wire rack to cool.
Mix the icing sugar with the boiling water and add the colouring, if using, then smooth over the biscuits or pipe icing details. Decorate with sweets and leave to set.