Step 1Line 8-inch-square baking pan with foil so that foil extends 2 inches beyond the 2 opposite sides of pan. Lightly butter bottom and sides.
Step 2Cook 1/2 cup butter, brown sugar and semisweet and unsweetened chocolate over low heat, stirring constantly, until butter and chocolate melt and mixture is smooth. Transfer to medium bowl.
Step 3With wooden spoon, stir in granulated sugar. Stir in eggs, 1 at a time, until there is no trace of yolk. Mix in vanilla. Add flour and salt and mix vigorously until mixture is shiny and smooth. Scrape batter into prepared pan and smooth top with rubber spatula.
Step 4Bake brownies at 350 degrees until toothpick inserted into center comes out with a few moist crumbs clinging to it, 35 to 40 minutes. Do not over-bake. Place pan on wire rack and cool 45 minutes. Using 2 ends of foil as handles, lift brownies out of pan. Invert onto cooling rack and peel off foil. Cool completely (brownies will be frosted on smooth side).
Step 1Heat butter and unsweetened and milk chocolate over very low heat, stirring constantly, until chocolate melts and mixture is smooth. Transfer to bowl of electric mixer. While mixing on low speed, add half of powdered sugar, dash salt and half of cream. Blend in remaining powdered sugar and remaining cream. Beat in vanilla. Beat frosting on medium speed until smooth and shiny, about 30 seconds.
Step 2Spread frosting over uncut brownies as smoothly as possible. Garnish top with chopped walnuts, if desired. Cut into 18 squares and serve at room temperature.