Step 1

Preheat the oven to 160°C. Grease and line the base of an 18cm square cake pan. Place the butter and chocolate in a heatproof bowl over a pan of simmering water (don't let bowl touch water). When butter and chocolate have melted, remove from heat and allow to cool slightly. Beat the sugar, vanilla and a good pinch of salt into the mixture. Whisk in the eggs one at a time, beating well after each addition, then add egg yolk and flour, beating until smooth.

Step 2

Fold in the walnuts and pour mixture into pan, smoothing the top. Bake for 40 minutes or until a skewer comes out with a few moist crumbs attached (not sticky batter). Allow to cool, then turn onto a wire rack and peel off lining paper. Cut into 12 neat squares and dust with cocoa before serving.