Preheat the oven to 160c fan/180c/350f/gas mark 4.
Melt the butter, sugar and chocolate in a bowl over a pan of simmering water. Make sure the bottom of the bowl does not touch the water. Take of the heat and leave to cool a little.
Beat in the eggs.
Mix the cocoa and salt into the flour, then pour into the chocolate mix and fold in until well combined.
Pour the mixture into a brownie tray and bake for 25 minutes.