Adjust a rack one-third up from the bottom of the oven and preheat to 350 degrees. Line a 13 X 9 X 5 cm pan with foil (turn pan upside down and mold foil to the pan, including up--down?--the sides, turn pan over and fit foil tightly into pan. Take a piece of butter, place it in the pan (on the foil), and place in the oven only until the butter is melted. Then, with crumpled wax paper, spread the butter all over the foil and set the pan aside. Sift together into a large bowl the cocoa, flour and salt and set aside. In a 3 litre heavy pan over low heat, melt the butter. Remove the pan from the heat and stir in the sugar, then the eggs two or three at a time, and then the vanilla. Add the butter mixture to the sifted dry ingredients, pouring it in all at once. Beat on low speed (of a mixer) or by hand, scraping the bowl as necessary, until ingredients are completely mixed. Stir in the walnuts. Turn the mixture into the prepared pan and bake for 50 to 60 minutes until a toothpick gently inserted in the middle comes out clean, but with a bit of chocolate clinging to it. Do not overbake or they will be dry. Cool in the pan at room temperature. When the bottom of the pan is just tepid, cover the pan with a length of wax paper and then with a flat cookie sheet or board. Turn the pan and the sheet over, then remove the pan and slowly peel off the foil. Leave the cake upside down and transfer to the refrigerator overnight. Cover the chilled cake with a cookie sheet or board and turn it over again, leaving the cake right side up. Remove the sheet and wax paper from the top of the cake. Using a long, strong sharp knife, cut the cake into 24 extra-large brownies (or smaller if you prefer). These are, supposedly, Maida Heatter's favorite brownies, the creation of Cristina Guiovanoli, then a student at the Culinary Institute of America These are very rich, obviously, and absolutely delicious.