From Baking for All Occasions by Flo Braker. Flo says: "Baking the silky, intense chocolate topping on crunchy (firm) shortbread makes for neatly cut brownies that will lend panache to any dessert time. I prefer to chop the nuts for this recipe in a nut mill for consistently even chopped nuts. For a more attractive finish, after you chop the nuts, sieve them to remove nut "dust" before topping the brownie." Don't be put off by the length of the recipe, it's just very detailed.