Preheat the oven to 180°C / Gas Mark 4. Line a 22cm square shallow tray with baking paper. Melt butter and chocolate in a bowl set over a pan of simmering water. Allow to cool.
Whisk eggs and sugars together until light and fluffy. Add chocolate mixture, stir well and sift flour, baking powder and cocoa into the bowl. Add nuts. Mix to combine and pour into the prepared tray. Bake 25-30 minutes until the top is cracked and the middle is just set. Cool completely and lift out of the tray.