Preheat the oven to 180C/350F/Gas 4. Line a baking tray with parchment paper or a silicone mat, and set aside.
Whisk together the flour, oats, baking powder, baking soda, cornflour, and salt. Set aside.
In a separate large bowl, whisk together the melted butter and sugars until combined.
Add the egg, vanilla, and peanut butter, and mix until combined.
Add the dry ingredients, and mix until just combined. Fold in the chocolate chips.
NOTE: The dough will be quite crumbly at this point. Add milk (1 tablespoon at a time) until the dough comes together and is more manageable. I usually need 3 - 5 tablespoons.
Roll the dough into 2-oz balls (or use a medium cookie scoop), and place onto the prepared baking tray. Bake for 10 - 12 minutes.
Allow to cool on the baking tray for 5 - 10 minutes before transferring them 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 straight from frozen.