TeamEdward recommended this one to me and it's utterly fab - I didn't bother with the frosting though, it doesn't need any:
"Preheat oven to 170C.
Line a 33x33x5 baking tin with greaseproof paper.
Ingredients:
5 medium eggs (I used 4 large eggs)
500g caster sugar
120g plain flour
100g cocoa powder
250g unsalted butter, melted
30g shelled walnuts, chopped
30g dark chocolate, roughly chopped
Put the eggs and sugar in a large bowl and beat with an electric handheld whisk until light & fluffy.
Add flour and cocoa and beat until all the. ingredients are well mixed.
Stir in melted butter until combined.
Stir in nuts and chocolate by hand until evenly distributed.
Spoon mixture into the prepared baking tray and bake in the preheated oven for about 30-35 minutes, or until the top is firm but the centre is still soft.
Leave to cool completely.
Frosting:
Beat together 200g icing sugar, 75g unsalted butter and 30g sifted cocoa powder in a freestanding electric mixer with a paddle attachment on a medium-slow speed until the mixture comes together and is well mixed. Add 150g cream cheese in one go and beat until it is completely incorporated. Turn mixer up to medium-high speed. Continue beating until the Frosting is light and fluffy, at least 5 minutes. Do not overbeat, as it can quickly become runny.
When brownie is completely cold, spread the frosting over the cake.
I left out the nuts and added a bit more chopped chocolate, because we have nut allergies in this house. It still worked fine."