Pre-heat your oven to 180ºC/350ºF and line up deeper baking tray with baking paper.
Start with making your cookie dough. Add egg and both sugars into a bowl and whisk by hand until pale in colour and increased in volume. Add vanilla, melted butter and whisk until combined. Add flour, salt, baking soda and fold with spatula until almost combined, add chocolate chunks and mix until all the flour incorporated and set aside. Do not over-mix as that would cause cakey cookies.
To prepare your brownie, add chocolate and butter into microwave safe bowl and melt in 30 seconds burst, steering in between, until the chocolate is fully melted and set aside. Whisk eggs and both sugars with electric whisk, or in standing mixer, until pale and fluffy, this step is very important, do not skip it. It will take 4-5 mins. Pour in chocolate mixure, vanilla and whisk until combined. Sift in flour, cocoa powder, salt and fold in with spatula, do not over-mix the batter.
To assemble the brookie, take half of the cookie dough, break into pieces and spread them evenly on the bottom of the prepared baking tray. Pour brownie batter over the cookie dough and spread pieces of the second half of the top of the brownie.
Bake in the pre-heated oven for 20-25 mins. When you insert skewer to test if it's done, some crumbs should stick to the skewer. It will still cook as it's cooling so it's better to underbake rather than over-bake. Let it cook completely before removing from the tray. You can chill the brookie in the fridge before slicing into bars with a sharp knife to have nice clean edge.