Preheat oven to 325 degrees. Lightly butter 13- by 9- by 2-inch metal baking pan. Melt butter and chocolate in heavy medium saucepan over low heat, stirring until chocolate mixture is smooth. Cool.
Using electric mixer, beat eggs and sugar in large bowl until light and fluffy. Beat in chocolate mixture, vanilla and salt. Add flour; beat just until blended. Pour batter into prepared pan, smoothing top. Sprinkle top with walnuts.
Bake until tester inserted into center comes out with crumbs attached, about 35 minutes. Transfer pan to rack; cool completely.