Preheat the oven to 180°C and thoroughly butter an 8 x 12 inch swiss roll tin.
Put the caster sugar and vanilla essence into a large bowl and pour the melted butter over. Stir thoroughly then beat the eggs into the mixture. Sieve the flour, cocoa and baking powder into the bowl, mix well and fold the roughly chopped walnuts through.
Pour and scrape the chocolate batter into the prepared tin and bake for 20-25 minutes until the brownies are just set in the middle. If your testing toothpick or skewer comes out with a couple of crumbs clinging to it, all the better. Cool on a rack before cutting - I find a pizza wheel works well.