American
Heat the oven to 180C and line a 20 cm square tin with baking paper.
Melt the butter and chocolate together in a pan over low heat, stirring, then take the pan off the heat and cool for 5 minutes.
Whisk in the sugar, then the eggs one at a time and the vanilla, until the mixture is thick and glossy.
Sift in the flour, cocoa and salt and fold gently until no dry flour remains.
Spread the batter evenly in the tin.
Bake for 22-25 minutes, until the top is set and a skewer comes out with moist crumbs.
Cool completely in the tin, then cut into 8 pieces.