Begin the salted caramel for these heavenly brownies at least 2 hours before you intend to bake, so that it has time to cool and firm up.
For the salted caramel, melt the sugar in a medium pan over a medium heat. Swirl the pan without stirring for 8–10 minutes, until the caramel has turned a dark amber colour.
In a separate pan bring the cream to the boil, then immediately remove it from the heat and set aside.
Remove the caramel from the heat and carefully add the salt and half the cream. (Take care as the mixture will spit.) Once the mixture has settled, add the remaining cream and the butter, and stir together. Return the pan to the heat and cook for 2 minutes, stirring, until thick and smooth.
Pour the salted caramel into a bowl. Leave to cool, then cover with cling film and chill for at least 2 hours, until firm but spoonable. When the caramel is almost ready, start the brownies. Heat the oven to 180°C/160°C fan/350°F/Gas 4.
Melt the chocolate and butter together in a bowl set over a pan of simmering water, stirring occasionally for 4–5 minutes, until smooth. Remove from the heat and set aside to cool.
Using an electric hand whisk, in a large bowl whisk together the eggs and both sugars until thick and pale. Whisk the cooled chocolate mixture and the vanilla into the egg mixture, then sift in the flour, cocoa powder and salt. Fold together until well mixed.
Pour the mixture into the prepared tin and level with the back of a spoon. Spoon the caramel onto the brownie mixture and sprinkle with a little sea salt.
Bake for 45–50 minutes, or until a skewer inserted into the centre comes out pasty (but not wet). Allow to cool in the tin completely, then remove from the tin and use a hot knife to cut it into squares.