Preheat the oven to 180°C (350°F). Butter and line the base of an 18 cm square tin with greaseproof paper.
Melt the butter and chocolate over a pan of simmering water. Remove from the heat and cool slightly. Stir in the sugar, salt and vanilla. Add the eggs, one at a time, stirring well until blended.Add the flour and cocoa and beat for 30 seconds until smooth.
Spoon the mixture into the tin and bake for 35 - 40 minutes. The mixture should still be slightly wet when tested with a skewer.
Cool in the tin for 10 minutes. Remove and cut into 10 servings.