In a medium mixing bowl, mix together the peanut butter and icing sugar until completely combined.

Roll the mixture into 0.5oz balls (you'll end up with about 15 - 16). Keep refrigerated whilst preparing the cookie dough.

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, cocoa powder, baking powder, baking soda, cornflour, and salt. Set aside.

In a separate large mixing bowl, whisk together the butter and sugars until combined.

Add the egg and vanilla, and mix until combined.

Add the dry ingredients and mix until completely combined.

Roll the dough into 1.5oz balls, and place onto the prepared baking tray.

Flatten each ball and place a peanut butter ball in the centre. Shape the cookie dough around the filling, covering completely, and roll back into a ball.

Bake for 12 - 14 minutes. As soon as the cookies are out of the oven, top with chocolate chips.

Allow to cool on the baking tray for 5 - 10 minutes before transferring them to a wire rack to cool completely.