First, make the brownie batter. Preheat oven to 190°C (170°C fan) and line a 20 cm square baking tin with nonstick baking paper

Put the melted butter and sugar into a large mixing bowl and beat together with an electric hand mixer until completely combined.

Add the eggs and continue to beat until the mixture is super thick, fluffy and pale, and has at least doubled in volume.

Mix in the melted dark chocolate until combined.

Sift in the flour, cocoa powder and salt, then fold in with a wooden spoon or rubber spatula until just combined.

Fold in the chocolate chips until evenly distributed. Set aside while you make the cookie dough.

Put the softened butter and sugar into a large mixing bowl and beat together with an electric hand mixer until light and fluffy.

Add the egg yolk and vanilla extract and beat until just combined.

Fold in the flour and baking powder with a wooden spoon or rubber spatula until a few streaks of flour remain. Then fold in the chocolate chips until evenly distributed.

Scoop the brownie batter into your prepared tin and smooth out evenly with the back of a spoon.

Scoop the cookie dough by the ½ tablespoonful and roll into a small ball with your hands, then repeat with the remaining dough.

Arrange the balls randomly over the top of the brownie batter, leaving gaps in between. Sprinkle over the extra chocolate chips.

Bake for 30 minutes until the edges are cracked and the cookie dough is golden brown and no longer wobbles in the middle.

Leave to cool completely in the tin. Chill in the refrigerator for at least 2 hours, or overnight, if possible, for a firmer texture and a cleaner cut.

Cut into 16 squares.Store in an airtight container at room temperature or in refrigerator for up to 1 week.