Pre-heat the oven to 350F / 180C. Line a 9-inch baking tin with baking paper, ensuring you fully cover the sides of the dish.
Break up the chocolate into pieces and place in a heatproof bowl.
Chop up the butter and add to the bowl.
EITHER microwave in 30 second-bursts, stirring each time until melted and smooth. OR place the bowl over a pan of simmering water and stir with a wooden spoon until melted and smooth and then remove from heat.
Stir in the vanilla extract and sugar.
Stir in the eggs and mix until smooth.
Add the flour and cocoa powder and stir until smooth.
Pour the mixture into the prepared baking tray and bake in the oven for 25 minutes.
Remove from oven and allow to cool completely in the tin.