Line a large baking tray with baking parchment. Beat the sugar and butter together in a large bowl until pale and smooth using an electric whisk. Pour in the egg, a little at a time, whisking well after each addition. Tip in the flour, cornflour, xanthan gum and salt, and beat again. (If using chocolate chips or citrus zest, stir this in now.) Flatten the dough into a disc in the bowl, then cover and chill for 1 hr.

Put the chilled dough between two sheets of baking parchment and roll out to 1cm thick. Using a 6cm cutter, stamp out as many biscuits as you can, arranging them on the tray with space between them. Re-roll the offcuts and repeat until all the dough has been used (you should get 12-15 biscuits). Chill for 30 mins.

Heat the oven to 180C/160C fan/gas 4. If you like, sprinkle the biscuits with a little demerara sugar, then bake for 8-10 mins until golden and firm at the edges. Leave to cool on the tray for 2-3 mins, then transfer to a wire rack to cool completely.