Preheat the oven to 190°C, fan 170°C, gas 5. Beat the butter and sugar together, then stir in the syrup. Tip in the flour and bicarbonate of soda, then mix to a soft dough. Chill in the fridge for 30 minutes.
Dust your work surface and your rolling pin with flour. Roll out the dough to a thickness of 5mm. Cut out your favourite Halloween shapes, using cutters or cut round templates of ghosts, monsters or bats. Alternatively, mould the shapes you like by hand. Transfer the shapes to a baking sheet lined with baking paper - allow plenty of space between biscuits (it may be easier to cook the biscuits in 2 batches). Bake for 8-10 minutes until golden brown and firm at the edges. Allow to cool on the baking sheet.
To make the icing, add enough water to the icing sugar to make a spoonable icing. Spread over the biscuits. Allow to set, then use the icing pens to make your biscuits look spooky!