Preheat your oven to 180ºc/160ºc fan and line a 9"x13″ traybake tin with parchment paper.
Melt together the unsalted butter and dark chocolate in a heatproof bowl over a pan of boiling water, on the microwave for 1-2 minutes until melted.
Leave to the side to cool to room temperature for 5-10 minutes
In a new bowl, whisk together the eggs and sugar for a few minutes until the colour has turned pale, is very mousse like, and is double the original volume
When whisked, pour the cooled chocolate mix over eggs and fold together carefully.
Once completely combined, add the cocoa powder and plain flour on top of the chocolate mix and then fold together again
Pour into the prepared tin and spread evenly
Bake in the oven for 20-25+ minutes until there is a small wobble in the middle.
Once baked, leave to cool in the tin completely. Then, set the brownies in the fridge for 3+ hours for best results.
Notes
These brownies will last for up to one week in the fridge or at room temperature on a cool day