Melt the butter and chocolate together in a heatproof bowl set over a pan of barely simmering water (make sure the base of the bowl doesn't touch the water). When the mixture is melted and smooth, lift the bowl off the pan and set aside to cool for 20min. Preheat oven to 180°C (160°C fan) mark 4 and lightly grease and line a 20.5cm square tin with baking parchment. Using a handheld electric whisk, beat the eggs and sugar together in a large bowl until thick and moussey - about 5min. Add the melted and cooled chocolate mixture to the egg bowl and fold together with a large metal spoon. Sift over the flour, cocoa powder and a pinch of salt and fold together. Scrape the mixture into the prepared tin, level and bake for 30min or until there is no wobble left when you gently shake the tin. Cool completely in the tin before dusting with icing sugar (if you like) and cutting into squares.