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Put the butter and sugar in a bowl and beat together with a whisk or electric beaters until smooth and fluffy. Tip in the plain flour, cocoa powder and food colouring, and mix again. Chill for 30 mins – this will make the dough easier to roll out. Lightly flour the work surface and roll out the dough until 1cm thick.
Cut out the biscuits using a person-shaped cutter, and arrange on 2-3 large baking trays. Reroll the excess dough and repeat. Chill the dough cut-outs for 30 mins.
Heat the oven to 180C/160C fan/gas 4 and bake for 18-21 mins, until hardening around the edges. Leave to cool fully.
Sift the icing sugar into a bowl and gradually pour in 1-1½ tbsp water, mixing constantly, to get a thick paste. Put the icing in a piping bag, snip a little off the end to make a small hole, and pipe bone shapes and a skull onto the biscuits so they look like skeletons. Leave to set. Will keep for up to a week in an airtight container.