Preheat the oven to 160℃ (320℉). Meanwhile line the brownie tin with greaseproof paper (parchment paper).
Fill the saucepan a quarter full with water and place onto the hob. Add the butter and chocolate to a mixing bowl and place onto the saucepan making sure the bottom of the bowl is not touching the water. Turn the hob to a medium heat and let the steam melt the butter and chocolate, stirring occasionally.
Once the butter and chocolate a completely melted add about half of the sea salt to it and stir in (reserve the second half for garnish). Keep the bowl to the side to cool slightly.
Meanwhile make a sabayon, into the stand mixer bowl add whole eggs and sugar, and whisk on a medium high speed until it thickens and volumises, it should form a ribbon when lifted.
While you are waiting for the eggs to thicken, sift the flour to remove any lumps.
Once the sabayon is ready, add the melted butter and chocolate in ⅓ increments gently folding with a spatula until fully incorporated. Make sure to scrape the sides and the bottom of the bowl until completely incorporated.
Fold the flour in a ⅓ at a time. Again scrape the sides and bottom of the bowl. Fold in the chocolate chips and spread evenly into the brownie tin. With the remaining sea salt sprinkle over the top for garnish and into the over for 25-30 minutes.1
Allow the brownies to cool completely before portioning into 16 pieces.2