Preheat oven to 350 degrees F (175 degrees C). Lightly grease an 8 inch square baking pan.
Melt butter and chocolate in top of a double boiler over simmering water, stirring frequently and scraping down the sides with a rubber spatula to avoid scorching. Remove top of double boiler and stir sugar, flour, eggs, salt, and vanilla extract into chocolate mixture until batter is smooth. Fold walnuts into batter; spread into the prepared pan.
Bake in the preheated oven until a toothpick inserted in the center comes out clean, about 25 minutes. Cut brownies while warm.
Brownies From Scratch · Dinner Match Lab | Dinner Match Lab