Beat the butter and sugar together with an electric whisk for 4-5 mins until light and fluffy. Beat in the egg until combined. Add the flour, cocoa powder, cinnamon and ginger, and continue to beat until the mixture comes together into a dough. Put the dough between two pieces of baking parchment and roll out to a 6mm thickness using a rolling pin.

Heat the oven to 200C/180C fan/gas 6. Remove the top layer of parchment and cut out cookies using a 6cm round or star cutter. Re-roll the offcuts and continue to cut out cookies until all the dough has been used (you should get 20-22, depending on the cutter).

Arrange the cookies on a large baking sheet lined with baking parchment, spacing them at least 2cm apart (you may need two baking sheets). Bake for 7-10 mins until slightly golden at the edges. Leave to cool on the baking sheet for 10 mins, then transfer to a wire rack to cool completely.

Once cool, make the icing. Beat the egg white with an electric whisk until just starting to froth up, about 1 min. Add the icing sugar, 1 tbsp at a time, whisking continuously at a medium-high speed until combined. Add the lemon juice and whisk to stiff peaks, about 5 mins – the icing should be shiny. Add more icing sugar if needed to help it stiffen up.

To decorate, divide the icing into bowls and colour with the gel food colourings (we left some icing white, then coloured three bowls with blue, yellow and red). Spoon the coloured icing into separate piping bags fitted with small round nozzles (or snip off the ends). Pipe lines on the biscuits to create firework-like ‘burst’ patterns or swirls. While the icing is wet, decorate with star sprinkles, if you like. Leave to set slightly, then serve. Will keep in an airtight container for up to a week.