Prepare a microwave safe, non-metallic tray by spraying it with cooking oil and ensuring it is fully coated.
Weigh out the butter into a microwave safe bowl and melt in the microwave on low.
Add the sugar, the cocoa powder, and then the vanilla extract and mix well.
Add each egg, one at a time, and mix in well after each. Stir in the flour and ensure it is all incorporated.
Spoon the mixture into the prepared tray and spread to the edges. Place in the microwave on high for 5 minutes. Then check it, it will probably need around 3 more minutes but keep an eye on it. It will be ready when it starts to shrink away from the edges. In the centre there may be a spot which looks uncooked but the mixture continues to cook while cooling so this will soon disappear.
Leave to cool completely and then cut in the tray. Transfer to a serving plate and tuck in!
Add chocolate chips or chopped chocolate for double chocolate brownies. Alternatively add nuts for a crunchy texture.