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.