Start by making the cookie dough. Place the butter and both sugars in a mixing bowl. Beat together until pale, light and fluff y; if you have a stand mixer, this will take 4–5 minutes. Add the vanilla and egg, then beat again.

In a large bowl, whisk together the flour, baking powder and salt. Add to the butter and egg mixture and fold through until combined. Add the chocolate chips and fold through again. Place the bowl of cookie dough in the refrigerator while you make the brownie dough.

For the brownies, in a large bowl, whisk together the flour, cocoa powder, baking powder and salt.

Place the chocolate chips and butter in a heatproof bowl set over a pan of barely simmering water, ensuring the base of the bowl doesn’t touch the water. Stir until melted and glossy. Set aside to cool.

Place the egg and both sugars in the a large mixing bowl or the bowl of a stand mixer. Whisk together until pale and voluminous.

Pour in the melted chocolate and fold through, then add the dry ingredients and fold again. Place the brownie dough in the refrigerator to chill for 15–20 minutes. Preheat the oven to 200°C/180°C fan/400°F/Gas 6.

Line two baking sheets with baking paper. Scoop out 1 tablespoonful of cookie dough, then scoop out 1 tablespoonful of the brownie dough. Combine the balls in your hands, gently rolling them together to join them. They should look marbled along the join but still clearly one side cookie and one side brownie. Place onto the baking sheets, then repeat to use up all the dough, ensuring you leave space between the cookies.

Bake the cookies for 12–14 minutes until puff ed up and golden at the edges but still soft in the middle.

Remove from the oven. Leave to cool for 10 minutes on the sheet before transferring to a wire rack to cool completely.