Preheat the oven to 160°C and line two baking trays with sheets of non-stick baking paper.
Place the softened butter and granulated sweetener in a medium mixing bowl and beat together until light and fluffy using an electric handheld whisk. This will take about 2 minutes. You can use a wooden spoon instead of an electric whisk, but it will take longer and require more effort.
Add the beaten egg and beat for another minute until completely mixed.
Sift the flour, cinnamon and baking powder and salt into the mixing bowl and mix thoroughly with a wooden spoon. Using clean hands, gather the crumbs together in the bowl and form into a ball of dough.
Lightly dust the work surface with a little flour and turn the dough out onto the surface. Knead a little until the ball of dough is smooth, but take care not to over handle otherwise the dough may become tough.
Roll the dough out on a lightly dusted surface to a thickness of about 2-3mm. Use an 8.5 x 6.5 cm gingerbread man cutter to cut out 24 biscuits. You will need to gather together the dough off-cuts and re-roll them to make 24 biscuits.
Place the biscuit shapes on the lined baking trays leaving space between each.
Place both baking trays of biscuits in the preheated oven and bake for 10-15 minutes until lightly golden. Check on the biscuits after 10 minutes to make sure they're not browning too much.
Remove the biscuits from the oven and leave to cool on the baking trays for 5 minutes, before transferring to a cooling rack to cool completely.
Sift the icing sugar into a small bowl. Add up to 3 tsp of cold water a little at a time and stir until you have a smooth, stiff icing. Take care not to add too much water otherwise the icing will be too thin to hold its shape when piped onto the biscuits.
Spoon the white icing into a piping bag fitted with a fine writing nozzle. Pipe simple skeleton shapes onto the completely cooled biscuits. Use a little readymade black writing icing to pipe small dots for eyes onto the skeletons. Leave to set and serve alone or as part of a selection of other Halloween treats.