For the salted caramel: Place the sugar into a heavy-bottomed pan and over a medium-high heat. Wait until the edges start to liquefy, then gently swirl the pan to dissolve all the sugar. This takes patience, so keep swirling until all the sugar is dissolved and its a deep golden-brown colour. Immediately take off the heat, then slowly whisk in 1/3 of the cream to stop the caramel from cooking. Then continue to slowly pour in the rest of the cream, whisking continually. Then add the butter and sea salt, mixing until smooth. Leave the caramel to cool slightly, then pour into a piping bag and put to the side until needed.
For the brownies: Preheat the oven to 180C/Gas Mark 4 and line a 20x34cm baking tray with baking paper. Add the eggs and sugar into a medium bowl and whisk a few times with an electric whisk to combine the two. Then put the butter into a medium pan and melt until bubbles start to form - being careful not to let the butter boil. Take off the heat and add the 54-60% chocolate and a third of the 70% chocolate mixing until smooth.
Whisk the eggs and sugar again, until it turns paler in colour, then add the chocolate mix. Sift in the flour and combine with a silicone spatula until completely combined. Finally, add the remaining chocolate, mix and then pour into the prepared baking tray.
Level out the brownie mixture with a spatula and then pipe lines of caramel lengthways across the brownie. Feather the caramel, by dragging a spatula down and up through the lines.
Pop in the oven for 22-25 minutes until baked through. The brownie will still feel soft to touch when done, so don't panic! Leave to cool in the tin and then cut into either 24 small squares.
Finally, if you want you can add more salted caramel to the cooked brownies for an extra caramel kick like I did!