Preheat oven to 325 degrees F.
In a medium saucepan, combine 6 ounces chocolate with butter, sugar, and water. Cook over low heat, stirring, until melted. Set aside to cool a bit. Break up the remaining chocolate and stir into melted chocolate mixture. Whisk in eggs, flour, and salt. Add walnuts if desired. Batter will be thick and lumpy. Spray a 9-inch square pan with non-stick spray and scrape batter into pan. Bake for 30 to 35 minutes. Set on rack to cool. Garnish with powdered sugar.
Recipe courtesy of Karen Moyer and Jamie Moyer, Seattle Mariners, from "Starters and Closers" by Karen Moyer, ed., benefiting the Fred Hutchinson Cancer Research Center