Preheat the oven to 200°C/180°C fan. Line a 10×8 inch/25×20 cm tray
In a bain marie melt the butter and chocolate together, gently stirring. Take the bowl off the bain marie and stir in the complete sugar until well combined.
Add the eggs, one at a time. Sift in the flour and combine, finally adding the walnuts. Pour everything into the prepared tray.
Bake on the middle shelf 20-25 minutes, let cool and serve.