Microwave butter and chocolate squares in a 1-quart microwave-safe bowl at HIGH 2 minutes or until both are melted, stirring once.
Beat eggs at medium speed with an electric mixer; gradually add sugar, beating well.
Add flour, salt, and vanilla, beating well. Stir in chocolate mixture, and, if desired, chopped walnuts. Pour batter into a lightly greased 13- x 9-inch pan.
Bake at 325° for 40 to 45 minutes. Cool in pan on a wire rack. Spread with frosting, if desired, and cut into squares.
Raspberry Brownies: Fold 1 cup fresh raspberries into Basic Brownies batter; pour into pan. Melt 1/2 cup raspberry jam; drizzle over batter, and swirl with a knife. Bake as directed. Sprinkle with sifted powdered sugar; cut into squares. Garnish with fresh raspberrries and raspberry leaves, if desired.
NOTE: For added interest, stir 1 cup of any of the following ingredients into Basic Brownie batter: butterscotch, peanut butter, semisweet or white chocolate morsels; candy-coated chocolate pieces; chopped candy bars or caramels; almond brickle chips; dried cherries. Bake as directed.