Preheat oven to 160°C and line a 8x8 inch baking tray with greaseproof paper...
Melt butter and dark chocolate in a heatproof bowl in the microwave until melted. Leave to cool to room temperature....
In a seperate bowl, whisk together the eggs & sugars for a few minutes until pale. Then pour the cooled chocolate into the eggs and fold together carefully.

Once combined, sift in the cocoa powder, plain flour and the chocolate chunks. Stir to combine then pour into the prepared tin. Top the brownies with nuts if you like. Bake in the oven for 25-30 minutes!