1. Heat oven to 325°. Grease an 8" x 8" baking pan with butter. Line the pan with parchment paper; grease the paper. Set the pan aside.

2. Melt the butter and the chocolate together in a 2-quart saucepan over low heat, stirring constantly with a wooden spoon. Remove the pan from heat and stir in the sugar. Add the eggs and vanilla and stir to make a smooth batter. Add the walnuts, flour, and salt; stir until incorporated. Pour the batter into the baking pan and spread evenly. Bake until a toothpick inserted into the center comes out clean, 40–45 minutes. Let cool on a rack. Cut and serve.