Step 1
Position rack in center of oven and preheat to 350°F. Line 9x9x2-inch metal baking pan with foil, leaving overhang. Spray foil with nonstick vegetable oil spray. Combine 6 ounces bittersweet chocolate and butter in medium metal bowl. Set bowl over saucepan of simmering water and stir until chocolate and butter are melted and smooth. Remove bowl from over water and cool chocolate mixture until lukewarm, 5 to 10 minutes.
Step 2
Whisk sugar, eggs, vanilla extract, and salt in large bowl to blend. Whisk in chocolate mixture. Stir in flour, then chopped toasted walnuts. Transfer batter to prepared baking pan. Bake brownies until tester inserted into center comes out with moist crumbs attached, 26 to 28 minutes. Transfer pan to cooling rack and let brownies cool completely.
Step 3
Bring cream to simmer in small saucepan over medium heat. Remove from heat. Add remaining 6 ounces chocolate to hot cream and let stand 5 minutes to soften, then whisk until melted and smooth. Pour chocolate ganache over brownie sheet in pan and spread to cover completely. Let stand at cool room temperature until topping is set, about 4 hours. DO AHEAD Can be made 1 day ahead. Cover and store at room temperature.
Step 4
Using foil as aid, lift brownie sheet from pan. Fold down foil edges. Using large sharp knife, cut brownie sheet into 25 squares, wiping knife with hot moist cloth after each cut. Arrange brownies on platter and serve.
