1. Heat oven to 350 degree F. Line a 13x9x2-inch baking pan with foil, letting foil extend at ends. Grease foil.
2. For the brownies, microwave the 1-1/2 cups butter on 100 percent power (High) in a large microwaveproof bowl 1 to 1-1/2 minutes or until melted. Whisk in cocoa until smooth; let stand 15 minutes to cool. Whisk granulated sugar into chocolate mixture until combined. Add eggs, one at a time, whisking after each addition, until blended. Stir in flour, 1 tablespoon vanilla and salt until blended. Stir in walnut pieces. Spread batter evenly into prepared pan.
3. Bake 45 to 50 minutes or until toothpick inserted in center comes out clean. Cool completely in pan on wire rack.
4. For the frosting, microwave chocolate and the 1/4 cup butter in a small microwaveproof bowl on 100 percent power (High ) 1-1/2 minutes; stir until smooth. Transfer mixture to a large bowl; add confectioners sugar, milk and 1 teaspoon vanilla extract. Beat on low speed of electric mixer to blend, then increase speed to medium-high and beat until frosting is smooth and spreadable.
5. Spread over Brownies. Using ends of foil, remove Brownies from pan; peel off foil. Cut into 24 squares. Makes 24 brownies.