Line a baking sheet with baking parchment. Lightly dust a clean work surface with flour and divide the dough into two. Beat the red food colouring into one batch of dough in a bowl using electric beaters until it’s vibrant red. Divide both lots of dough into two pieces each and chill them for 15 mins.

On the dusted work surface, roll out into four 1cm-thick rectangles, keeping them as straight-edged as possible – a dough scraper or palette knife can help with this. Lay the sheets of dough on top of each other, alternating colours and chill in the fridge until firm, around 30 mins. Trim the edges and cut into strips 1cm thick, then lay the strips on top of each other, alternating the colours, so that a red strip sits on top of plain and vice versa, and brushing lightly with a bit of water to help them stick. Put on a baking tray and chill in the fridge for 1 hr.

Heat the oven to 200C/180C fan/gas 6 and line two large baking sheets with baking parchment. Using a very sharp knife, cut into squares and lightly brush the edges with water. Dip the edges in the demerara sugar then arrange on a baking tray. Bake for 8-10 mins until golden and set. Leave to cool on the tray for 10 mins, before removing to a wire rack to cool completely. Will keep in an airtight container for three days.