Heat the oven to 170C, 150C fan oven, gas mark 3. Lightly grease two baking sheets.
Put the butter into a medium bowl. In a small pan, gently heat together the sugar, syrup, caramel, bicarbonate of soda and mixed spice, until smooth. Bring to the boil and pour over the butter. Stir until all the butter has melted.
Beat in the egg yolk and flour and knead lightly on a floured surface to a smooth dough. Roll out the dough to about the thickness of a pound coin. Use a variety of cutters to cut out the cookies; we used large and small snowflake-shaped ones.
Transfer to the baking sheets and bake for 10-12 minutes. Cool completely then store in airtight jars.
The thinner the dough, the crisper the biscuits will bake. If possible, roll out the dough between sheets of non-stick baking parchment.
For tree decorations, make a hole in the top with a skewer when the biscuits are hot out the oven and thread with fine ribbon when cold.
The biscuits will store for 2-3 weeks in an airtight container or for 1-2 months in the freezer.