Preheat oven to 350 degrees and butter and flour a 9-inch square baking pan, knocking out excess flour.
In a metal bowl set over a pan of barely simmering water melt chocolates and butter, stirring, and remove bowl from heat. Cool chocolate mixture.
In a large bowl whisk together sugar, eggs, and vanilla until smooth and whisk in chocolate mixture. Add flour and salt and stir until combined well. Stir in walnuts and spread batter in prepared pan. Smooth top and bake in middle of oven 30 to 40 minutes, or until a tester comes out with crumbs adhering. Cool brownies in pan on a rack and cut into 3-inch squares.