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.