Heat oven to 180C/160C fan/gas 4 and line 2 baking sheets with baking parchment. Put the butter and icing sugar in a large bowl and beat with an electric hand whisk for about 5 mins until pale and fluffy. Add the vanilla extract and beat again until fully incorporated.
Sift in the flour, cornflour and baking powder, and fold into the mixture using a spatula until combined (the dough should have a tacky consistency). Spoon the dough into a piping bag fitted with a large star-shaped nozzle. If all the mixture doesn’t fit, do it in 2 batches.
Pipe swirly circles 5cm diameter onto 2 baking sheets making sure there are 3cm spaces between each swirl.
Bake for 10-12 mins, swapping the trays over halfway through the cooking time so the biscuits are evenly baked, until pale golden and cooked through. Leave to cool on the baking sheets for a few mins, then transfer to wire racks.