Pre heat the oven to 175 C | 160 Fan | Gas 5 and line a 20cm square tin with baking paper.
Pop the dark chocolate, butter, vanilla extract and coffee granules into a large pan and place over a low heat on the hob (lowest possible setting). Stir constantly until the chocolate has nearly melted, then remove from the heat and continue to stir until completely melted. Make sure you scrape the bottom of the pan and sides when stirring to stop the chocolate from burning. Do not rush this stage.
Stir in the sugar until well combined.
Stir in the eggs and additional yolk one at a time until combined. Do not beat in the eggs, we don't want to aerate the mixture.
Fold in the flour and cocoa powder until just combined.
Scrape the brownie batter out of the pan into the prepared square tin and use a spatula to level out the mixture. Place in the centre of the oven and bake for 25-30 minutes. Test the brownie by poking a tooth pick in the centre of the brownie. Unlike a cake the tooth pick should have some brownie crumbs stuck to it. If the tooth pick comes out with runny brownie batter on it pop it back in the over for a little longer. It's always better to slightly under bake a brownie than over bake.
When the brownie is ready remove it from the oven. Allow the brownie to cool in the tin for 5-10 minutes. Then carefully remove it from the tin and place on a cooling rack to cool to room temperature.