I use a 12x8 baking tray for this quantity. Line it with non stick parchment paper to make sure nothing sticks.
Preheat the oven to 170C/150C Fan/Gas 3. L
Place a sieve over a mixing bowl and sieve the cocoa, flour and baking powder together and add the pinch of salt.
Break 80%/250g of the chocolate and add to a heat resistant bowl.
Place a saucepan on a medium high heat and add water. Bring to a simmer. NOTE: the water should not be high enough to touch the chocolate bowl.
Place the chocolate bowl over the simmering water and allow to melt, stirring regularly.
Once the chocolate has fully melted, remove from the heat and put to one side to cool slightly.
Bash the remaining chocolate into pieces and put to one side for now. Try not to eat it!!
In a separate bowl, add the sugar and softened butter and beat together until light and airy.
Add the eggs one at a time and gently beat in.
Add the melted chocolate and mix thoroughly until fully combined.
Add approximately one third of the flour and mix together. When combined add another third and so on.
Add about half of the nuts and the remaining chocolate (this will melt to forma a gooey middle.) and mix again.
Transfer from the bowl to the pre lined baking tray making sure you scrape all of the bowl. Don't want to miss any.
Sprinkle the remaining half of the nuts over the top of the mixture.
Place into the preheated oven and bake for about 25 - 30 minutes. To test the brownies, push a skewer into the cake, if it comes out wet then it is not cooked but a few crumbs on it means they're ready.
Remove the tray from the oven and allow to cool. Cut into square slices and place in an airtight container.