Preheat the oven to 180°C, fan 160°C, gas mark 4. Grease and line 32 x 22 x 5cm rectangular baking tray with baking paper.
Put the chocolate and butter in a bowl set over a pan of simmering water for about 5 minutes, until melted. To melt in the microwave, place in a microwave-proof jug and melt for 2 minutes on full power.
Put the sugar and eggs in a bowl and whisk with a hand-held electric whisk until light and pale.
Sift in the flour and cocoa. Add the melted chocolate mixture and fold the ingredients together until smooth.
Pour into the prepared tin and bake for 30-35 minutes. Allow to cool completely in the tin before turning out and cutting into 16 squares.