Preparation Preheat oven to 350°F. Butter and flour 13x9x2-inch metal baking pan. Stir butter and chocolate in heavy small saucepan over low heat until melted and smooth. Cool to lukewarm. Whisk flour, salt, and baking soda in small bowl to blend. Using electric mixer, beat sugar, eggs, and vanilla in large bowl until thick and billowy, about 3 minutes. Beat in chocolate mixture, then flour mixture. Fold in walnuts. Transfer to pan. Bake brownies until puffed and tester inserted into center comes out with a few moist crumbs attached, about 28 minutes. Sprinkle brownies evenly with chopped toffee bars. Cool in pan on rack. (Brownies can be made 1 day ahead. Cover; store at room temperature.) Cut into squares.
Preparation Preheat oven to 350°F. Butter and flour 13x9x2-inch metal baking pan. Stir butter and chocolate in heavy small saucepan over low heat until melted and smooth. Cool to lukewarm. Whisk flour, salt, and baking soda in small bowl to blend. Using electric mixer, beat sugar, eggs, and vanilla in large bowl until thick and billowy, about 3 minutes. Beat in chocolate mixture, then flour mixture. Fold in walnuts. Transfer to pan. Bake brownies until puffed and tester inserted into center comes out with a few moist crumbs attached, about 28 minutes. Sprinkle brownies evenly with chopped toffee bars. Cool in pan on rack. (Brownies can be made 1 day ahead. Cover; store at room temperature.) Cut into squares.