Preheat the oven to 180C. Grease and line a 9 inch ceramic tray or cake tin. Line with parchment paper and set aside.
Melt the dark chocolate and butter together in a bowl set over simmering water.
Beat the eggs and the sugar together until light and fluffy. Fold in the melted chocolate then, fold in sieved flour. Add the chocolate chips and nuts and pour the mixture in the prepared tin. Do not over mix as the air will be knocked out of the mixture.
Pour the mixture into the prepared tin and cook for 25-35 minutes until the centre of the cake feels firm on the surface but with an element of wobble underneath.
Allow the cake to cool slightly in the tin and then after a few minutes remove from the tin and cut into even sized squares.
Serve within 2 -3 days.
