Heat the oven to 200°C (180°C in a fan oven), gas 6 and grease a baking tin.
Beat together the butter, sugar and orange zest and stir in the eggs one by one.
Mix together the flour, cocoa powder and the walnuts and stir into the egg mixture.
Pour the mixture in the tin and spread evenly.
Bake for 30-45 minutes, remove from the oven and leave to cool.
Melt the couverture in a bain marie and spread on top of the cake.
Cut into 12 brownies and leave to firm.