Grease a shallow 20-25cm baking tin and preheat the oven to 180 C / Gas 4.
Break up the cooking chocolate and cube the butter and put in a heatproof bowl above a saucepan of simmering boiling water. Stir until the chocolate and butter have melted, then remove from heat and set aside.
In a large mixing bowl combine the chocolate mixture with the sugar, then gradually beat in the two whole eggs and one egg yolk. Finally add the vanilla and stir until well combined.
Sift in the flour and cocoa powder and fold together gently without over-mixing. Tip the mixture in the prepared baking tin.
Bake for approximately 25 minutes until the centre of the brownie is just set.
Remove from the oven and place on a rack for at least 30 minutes before removing from the tin and slicing into 10 squares. Eat them once they have slightly cooled if you prefer sticky and moist brownies, if you want them rich and dense leave them to cool completely overnight.
Using light brown sugar gives the brownie a richer and slightly caramelised flavour.