Preheat oven to 350°. In a small saucepan, melt butter and chocolate over low heat. Cool slightly., In a large bowl, beat eggs, sugar, vanilla and salt until blended. Stir in chocolate mixture. Add flour, mixing well. Stir in walnuts. , Spread into a greased 9-in. square baking pan. Bake 35-40 minutes or until a toothpick inserted in center comes out with moist crumbs (do not overbake)., Cool completely in pan on a wire rack. If desired, dust with confectioners' sugar. Cut into bars.