Sure to be met with plenty of ooohs and aaahs, these fragrant, spiced biscuits, decorated to look like fireworks, are the perfect treat for a bonfire night party.
For the biscuits, beat together both butters in a stand mixer fitted with the beater, on medium speed for 1 minute, until combined. Add the sugar and beat again, on medium speed for 2–3 minutes, until light and fluffy. Add both zests, along with the egg and cardamom and beat again for 2–3 minutes to fully combine.
Sift in the flour and bicarbonate of soda and use a wooden spoon to mix together well. Using your hands, bring the mixture together into a dough. Place the dough in the fridge to chill for 30 minutes.
Once the dough has chilled, place a sheet of baking paper on your work surface, dust with flour and tip out the dough onto it. Flatten the dough into a rough circle with your hands and place the second piece of baking paper on top. Roll the chilled dough between the two sheets of paper until it is 4mm thick.
Using the fluted, round cutter, cut out 12–16 biscuits, transferring them to the lined baking sheets, spaced well apart, and re-rolling the trimmings as necessary. Chill the biscuits for 30 minutes.
Meanwhile, heat the oven to 190°C/170°C fan/375°F/Gas 5 and make the royal icing.
Using a balloon whisk, whisk together the egg whites and 6 tablespoons of the icing sugar in a bowl, until smooth. Add the remaining icing sugar in batches, whisking to combine each time, until you have a stiff consistency.
Spoon one third of the icing into a separate bowl and colour it black. Divide the black icing equally between two bowls and loosen one portion with a few drops of water to give a flooding consistency. Set aside.
Divide the remaining icing between three bowls. Keep one portion white, colour another portion red, and colour the final portion yellow. Spoon each portion (the two blacks, and the white, red and yellow) into its own piping bag and set aside.
Remove the biscuits from the fridge and bake them for 10–12 minutes, until golden. Leave to cool on the baking sheets for 5–10 minutes, then transfer to a wire rack to cool completely.
Snip a tiny hole in the tip of the piping bag containing the thick black icing and pipe a fluted border around the edge of each biscuit. Snip a small hole in the piping bag containing the flooding black icing and fill inside each border, covering the biscuits (use the toothpick to help flood the icing evenly and pop any air bubbles). Leave to set for at least 1 hour.
Cut a tiny hole in each bag of white, red and yellow icings and pipe fine lines radiating from the centre of each biscuit – begin with the white, then fill in with lines of the red and yellow until your decorations resemble starburst fireworks.
Leave the biscuits to dry, then, using the cake-decorating paintbrush, add touches of gold paint to highlight the lines in places, giving your biscuits some celebratory sparkle.