Put the butter, sugar and peppermint essence in a food processor and whizz until pale and creamy. Add the flour, cornflour and salt, and pulse until the mixture clumps together into small pieces. Tip onto a lightly floured work surface and bring the dough together as a ball, but don’t overwork it. Roll the dough to a 25cm circle, about 1cm thick. Trim around a large plate to give a really neat edge. Transfer to a large baking sheet. Use 2 fingers to crimp all the way around the edge of the dough then, using a fork, mark dotted lines to portion the shortbread into 8 wedges. Put the tray in the fridge and chill for at least 30 mins.
Heat oven to 180C/160C fan/gas 4. Cook the shortbread for 25 mins until golden and cooked through. Leave to cool completely on the tray. To decorate, place a lacy doily over the shortbread and dust with a generous layer of icing sugar. Pull the doily away to reveal the beautiful lacy pattern. Keep in a tin for up to 5 days.