Preheat the oven to 160ºC . Butter an 20 cm square baking pan. In a heavy saucepan, melt the chocolate with the butter over low heat, stirring until completely melted. Remove from the heat and stir in the sugar. Add the eggs and vanilla and "beat it all like mad". Stir in the flour, salt and walnuts and mix well. Spoon the batter into the prepared pan and bake for 40 minutes. "Take it out; let it cool; cut into squares and go crazy."