In a large mixing bowl, beat together the butter and caster sugar until pale and fluffy, about 3-5 minutes. Add in the egg and lemon (or vanilla) extract and beat again until combined.
110 g (½ cup) unsalted butter, 100 g (¼ cup + 3 tbsp) caster sugar, 1 large egg, 1 tsp lemon (or vanilla) extract
Sift in the flour, baking powder and salt. Beat again until the mixture comes together into a stiff dough.
280 g (2 cups) plain (all purpose) flour, 0.25 tsp baking powder, 0.25 tsp salt
Press the dough together, shaping into a flat disc. Wrap it tightly in cling film and place in the fridge to chill for at least two hours.
Once the dough has chilled, preheat the oven to 160C/320F and line two baking trays with parchment paper.
On a lightly floured surface, roll the dough out to roughly ¼" thickness and stamp out biscuits using a cookie cutter. Place them on the prepared baking tray, leaving a little space between for even baking.
Bake the biscuits in the preheated oven for 9-10 minutes until firm and the edges are just turning golden. Cool for a few minutes on the baking tray, then transfer to a cooling rack to cool completely before assembling.