Line a large baking tray with parchment paper, or a silicone mat, and set aside.
Whisk together the flour, baking powder, baking soda, cornflour, and salt. Set aside.
Whisk together the melted butter and sugars until combined. Add the egg, peppermint extract, and green food colouring (enough to reach desired colour), and mix until combined.
Add the dry ingredients, and mix until just combined. Fold in the chocolate chips.
Roll the dough into 1.5oz balls, and place onto the prepared baking tray. Freeze for 1 hour.
Preheat the oven to 180C/350F/Gas 4.
Bake for 10 - 12 minutes, or until barely starting to brown around the edges. Allow to cool on the baking tray for 5 - 10 minutes before transferring to a wire rack to cool completely.
Video
Notes
Cookies stay fresh, kept in an airtight container, at room temperature for up to 1 week.Cookie dough balls can be kept in the fridge for up to 5 days, or frozen for up to 3 months. Bake from frozen.