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, cocoa powder, baking powder, cornflour, baking soda, and salt. Set aside.
Whisk together the butter, cream cheese, and sugar. Add the egg and mix until combined, then mix in the vanilla extract.
Fold in the dry ingredients, then fold in the chocolate chips.
Roll the dough into 1.5oz balls, about 18 - 20 balls, and place on the prepared baking tray. Flatten slightly, then press the extra chocolate chips on top.
Bake for 8 - 10 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.