Dry-fry the walnuts, cool, then chop them roughly.
Chop the chocolate, then put into a saucepan with the butter and sugar and cook over low heat, stirring, until melted. Let it cool.
Preheat the oven to 180°C.
Lightly beat the eggs, add to the chocolate & butter mixture. Add the remaining ingredients, combine well.
Grease a 22 x 22 cm (approx) baking tin and line the base with baking paper.
Transfer the mixture into the tin spread it out evenly.
Bake for about 25 minutes - it's done when it's just set (test with a skewer - the middle of the cake should remain a bit gooey & sticky).
Allow the cake to cool in the tin, then turn it out onto a plate, carefully slice into 12 squares. Sprinkle with cocoa powder.