Pre heat oven to 150°C/ 300°F or 130°C fan
Spray a 20cm (8 inch) square pan with cooking spray and line it with baking paper
In a large mixing bowl, add the melted butter and sugars, then whisk to combine.
Add the eggs, then whisk well for a good 2 minutes.
Sift over the flour and cocoa, then fold through until almost combined
Add the chocolate chips and nuts, then fold through. Be careful not to overmix!
Pour the batter into the prepared pan and smooth if necessary (you can just give the pan a wiggle, or you can use a spatula to spread the mixture)
Bake for 45 minutes, until the edges are slightly crispy, but the centre is still a little gooey. The brownie will continue to cook as it cools in the pan.
Remove the brownie from the oven and allow it to cool completely before slicing into squares.