Preheat oven to 350.
Heat the chocolate and butter in the saucepan until almost completely melted. Remove from heat.
Beat eggs and salt until foamy. Gradually whisk in sugar until thickened. Add vanilla and chocolate, then flour, then walnuts.
Butter the baking pan and spread the brownie batter into it. Bake for 25 minutes, then insert a toothpick into the center. If it comes out nearly clean, the brownies are ready. If it’s very chocolately, bake another 5 minutes.
Cool, cut, dust with powdered sugar, and serve.