Pre-heat oven to 180°C /160°C fan /gas mark 4/ 350°F.
Line and grease a 6 inch square cake tin.
Break up 90g of the dark chocolate (the extra 30g will be used later).
Melt the broken up chocolate with the butter in a bain marie or a glass bowl set over a saucepan of simmering water.
Remove from the heat and allow to cool whilst you are preparing the rest of the ingredients.
Whisk the sugar with the eggs, vanilla extract and salt in a large bowl with an electric whisk until light and fluffy. This should take about 3 minutes.
Pour in the melted chocolate and fold in with a metal spoon until just combined.
Whisk the cocoa powder in with the flour and fold into the rest of the ingredients until the mixture is thick and just combined.
Pour the brownie batter into the pan. Chop the extra 30g of chocolate roughly and scatter over the brownies.
Bake for 30 minutes.
Remove from the oven and leave the brownies to cool in the tin.
Once the brownies are cool, remove from the tin and cut into 9.
Sprinkle over extra sea salt if you like.