Preheat the oven to 180 C / Gas 4. Grease and line a brownie tin with baking parchment.
Melt the butter in a saucepan over a low heat. Pour into a big mixing bowl and add the sugar and mix well. Add the eggs a little at a time and beat well. Sift the flour, baking powder and cocoa powder and mix in. Add the walnuts and finally mix in. Pour into the prepared tin.
Bake in the preheated oven for 40 minutes or until a skewer inserted into the centre comes out clean. Remove from the oven and cool for ten minutes. Dust with icing sugar and serve!