Sift the plain flour, bicarbonate of soda and mixed spice in a bowl and stir together. Rub in the butter until the mixture resembles breadcrumbs.
Stir in the ginger, sugar, egg and golden syrup until you have a firm dough. Turn out on to a floured work surface and knead briefly until smooth. Form into a ball, wrap in clingfilm and chill for 15 minutes.
Meanwhile, preheat the oven to 180°C, fan 160°C, gas 4, and line 2 baking trays with baking parchment.
Roll out the dough on a lightly floured surface to 5mm thick. Stamp out snowflake shapes with the cutters (re-rolling and cutting the trimmings). Space out well on the baking trays. With the end of the straw, make a hole in the top of each cookie.
Bake for 10-12 minutes until light golden brown. Cool on the trays for 5 minutes, then transfer to a wire rack to cool completely.
To decorate, mix the icing sugar and 1-2 teaspoons of water in a bowl until smooth and glossy. The mixture should coat the back of a spoon; add more water if needed.
Spoon the icing into the piping bag and snip off the tip to make a small hole. Ice snowflake designs on the cookies, then sprinkle with edible lustre. Keep in an airtight container for up to 3 days. To hang as decorations, thread thin ribbon through the holes and tie to your Christmas tree.