In a small mixing bowl, whisk together the butter, sugar, cocoa powder, milk, and vanilla extract until smooth and combined.

Add the flour and gently fold until fully combined.

Scoop the brownie batter into 13 portions of approximately 14g each onto a lined baking tray. Place in the freezer whilst preparing the cookie dough.

Preheat the oven to 190°C/170°C Fan/375°F/Gas 5. Line a baking tray with parchment paper or a silicone baking mat. Set aside.

In a medium mixing bowl, whisk together the flour, baking powder, baking soda, cornflour, and salt. Set aside.

In a separate large mixing bowl, whisk together the butter, light brown sugar, and caster sugar until combined.

Add the egg and vanilla extract, and whisk until smooth and combined.

Add the dry ingredients and mix until just combined. Gently fold in the chocolate chips.

Divide the cookie dough into 13 portions of approximately 56g each. Flatten each portion slightly, keeping the centre of the base thick.

Place a frozen brownie batter portion into the centre of each piece of cookie dough.

Carefully fold the cookie dough up and around the brownie filling, stretching the dough from the sides and pinching it together at the top to seal completely. Make sure the dough at the bottom remains thick.

Place the stuffed cookie dough balls onto the prepared baking tray and freeze for 10–20 minutes, or until firm.

Bake for 12 minutes, or until the edges are set and the tops are lightly golden. The centres will still be soft.

Allow the cookies to cool on the baking tray for at least 10–20 minutes before transferring them to a wire rack to cool completely. The brownie centres will be very soft at first but will firm up as the cookies cool.