1. Heat over to 325; Grease an 8 x 8 pan with butter. Set aside.
2. Melt the butter and chocolate in a 2 qt. saucepan over low, stirring constantly. Remove from heat and stir in sugar. Add the eggs and vanilla and stir to make a smooth batter. Add walnuts, flour, and salt; stir until incorporated. Pour the batter into the baking pan and spread evenly.
3. Bake until a toothpick inserted in center comes clean, 40-45 minutes. Let cool on a rack. Cut and serve.