These dark and spicy biscuits have a real snap. They keep well for Christmas and can be decorated with sugar paste in any design you like. They are a perfect family activity in the run-up to the festivities.
Make the biscuits. Beat the butter and sugar in the bowl of a stand mixer fitted with the beater until light and creamy. Beat in the treacle and egg yolk followed by all the dry ingredients until you have a soft dough.
Turn out the dough onto a work surface and gather it into a ball, then wrap it in cling film and refrigerate for 30 minutes until firm enough to roll.
Heat the oven to 180°C/160°C fan/Gas 4. Roll out the chilled dough on a lightly floured surface to about 5mm thick, then cut out a variety of shapes with cutters. Place the biscuits on the lined baking trays and bake for 10–12 minutes, depending on size, until they just darken a little at the edges. Allow to cool on the trays for 5 minutes, then transfer to a wire rack to cool completely.
To decorate, dust the work surface with a little icing sugar and knead the sugar paste to soften. Roll out the sugar paste to about 3mm thick and cut out shapes with the biscuit cutters to match the biscuits. Brush the biscuits with a tiny amount of edible glue and stick the matching shapes of sugar paste on top.
Make a royal icing. Put the egg whites in the bowl of the stand mixer fitted with the whisk. Whisk for 10–20 seconds at medium speed, then turn off the motor and add some of the icing sugar. Whisk at a slow speed, gradually adding the remaining icing sugar, increasing the speed as the icing sugar is incorporated.
Use a spatula to wipe any mixture from the sides of the bowl, then add the lemon juice. Keep beating at high speed until you have a piping consistency.
Transfer the icing to the small piping bag with the nozzle and pipe designs onto the biscuits, then sprinkle with edible glitter or lustre powder.