Whisk in the cocoa powder and leave to cool a little.
In a large bowl whisk the eggs with the sugar and vanilla extract until everything is well mixed.
Fold in the flour.
Carefully stir in the butter mixture followed by the walnuts until just incorporated.
Spoon into an 8″ (20cm) sq silicone cake mould or lined tin and bake at 180℃ (350℉, Gas 4) for about 30 minutes. The brownie batter should be well risen with a crust over the top, but with a slight wobble to it. I tend to err on the side of caution and bake them less rather than more as I can’t stand overdone dry brownies.
Leave to cool then cut into 16 squares.
Please note: calories and other nutritional information are per serving. They're approximate and will depend on exact ingredients used.