Whisk together the butter, brown sugar and egg until paler in colour. You can also use an electric mixer.
Add the vanilla essence, flour and fine salt and incorporate using a spatula.
Fold in the dark chocolate chunks leaving a few for the top.
Line a 20x20cm mould with baking paper and pour the mix in. Level it using a spatula and sprinkle extra chocolate and coarse salt on top.
Take to a preheated oven at 180C for 15min.