Preheat the oven to 180 degrees celsius and prepare your baking tin by greasing with a little butter and then lining with greaseproof parchment paper.
Melt the butter in a medium based saucepan, it won’t take long to smell nutty and have brown bits in it, which sink to the bottom. Don’t leave the butter as it can go black and burn quickly. Once it’s at the nutty stage, remove from the heat immediately and decant into a medium bowl to stop the cooking and cool for 5 or so minutes. You want the butter to remain a little warm so that it melts the chocolate a bit, creating a marble effect.
Sieve the flour into a medium bowl and add the salt, whisking to distribute evenly.
In another medium bowl, whisk the eggs and vanilla.
In a large bowl add the brown sugar, pour in the melted butter and mix with a wooden spoon till thoroughly combined and glossy.
Add the whisked eggs to the butter/sugarmixture, stir with a wooden spoon until incorporated.
Chop up your chocolate into chocolate chip size pieces, leaving some a bit larger. When you add the chocolate there will naturally be some much smaller shavings/crumbs- don’t add these as they taint the colour of the mix and create a muddy colour.
Stir in the flour 3/4 of the way, then add in the chocolate and stir again until everything is well combined.
Spoon in the mixture into your baking tin and spread out using your wooden spoon until it’s an even thickness. I like to sprinkle a little extra sea salt on top before baking but this is completely up to you.
Cook in the pre- heated oven for 25 minutes at 180 degrees until you have a crispy top and browning round the edges.
It will set up a lot as it cools, and this is something you definitely don’t want overcooked!
Leave in the baking tray to cool completely and then remove from the tin and cut into 24 squares.