Preheat oven to 350°F. Line an 8-inch baking pan with nonstick foil.
In a medium pan, melt the butter and chocolate over very low heat. Stir until smooth; remove from heat. Whisk in sugar, eggs, and vanilla.
In a small bowl, combine the flour, cocoa powder, and salt. Stir into chocolate mixture until combined. Fold in walnuts.
Pour batter into prepared pan and bake in preheated oven until a pick inserted in the center comes out with moist crumbs, about 30 minutes. Let cool completely in pan before cutting.
Nutrition Info:
265 calories (4g protein, 24g carbohydrate, 17g fat) per serving