Melt chocolate with butter in top of double boiler over hot water. Beat eggs, sugar, salt and vanilla in large mixer bowl. Add melted chocolate mixture. Mix until blended. Add flour gradually while beating on low speed. Mix in nuts. Turn into 2 greased 18x 12-inch baking pans, spreading mixture evenly in pans. Bake at 350F 20 to 25 minutes, being careful not to overbake. While still warm, cut into squares.