Scoop out 16 teaspoons of peanut butter, and place them on a baking tray lined with parchment paper or a silicone mat. Freeze until solid - at least 1 hour.
Preheat the oven to 180C/350F/Gas 4, and line a baking tray with parchment paper, or a silicone mat. Set aside.
Whisk together the flour, baking powder, cornflour, baking soda, and salt. Set aside.
Whisk together the 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.
Roll the dough into 2oz balls, and flatten them out. Place the frozen peanut butter into the centres, then roll the dough back into balls. Place onto the prepared baking tray, and press the extra chocolate chips on top.
Bake for 10 - 12 minutes. Do not over-bake. Allow to cool on the baking tray for 5 - 10 minutes before transferring to a wire rack to cool completely.
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 for an extra minute.