Melt the butter and 185g of chocolate in a bowl over a pan of simmering hot water. Once melted, leave to the side to cool down.
Pre heat oven to 180C/ 160C fan / Gas 4. Line a 20cm square tin with baking parchment.
Sieve flour and cocoa powder In a separate bowl to get rid of the lumps.
Chop some chocolate into small chunks, any chocolate is fine.
Break the 3 eggs into a separate bowl and add the castor sugar. Use an electric whisk on maximum speed until its thickened up. This will take 5-10 minutes depending on the speed of the whisk. It should double in size and become a pale colour when ready.
Pour the cool chocolate mixture Into the whisked egg mix and fold together until they are mixed together. Make sure not to fold for too long as this will knock all the air out of the eggy mixture.
Next, pour the flour and cocoa powder mix into the chocolate mix and gently fold together again. Be patient with it until it looks creamy and ready.
Finally, mix in the chocolate chunks until evenly spread in the mixture.
Pour into the tin and put in the oven. Set a timer for 25 minutes. After the timer, take the tin out and slightly shake, if it still wobbles then bake for another 5 minutes until baked. Once it’s out the oven, leave for an hour before cutting it as it will break and crumble if cut too early.