Position rack in center of oven and preheat to 350°F. Spray a 9 x 9 x 2-inch baking pan and cover with parchment paper, leaving an overhang on two sides so you can lift the brownies from the pan. Lightly spray the parchment paper with nonstick cooking 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. Set aside.

Meanwhile, whisk sugar, eggs, vanilla extract, and salt in large bowl until combined. Slowly, whisk in chocolate mixture. Stir in flour, then chopped toasted walnuts. Transfer batter to prepared baking pan.

Bake brownies for 25-35 minutes or until tester inserted into the center comes out with only moist crumbs attached. Transfer pan to cooling rack and let brownies cool completely.

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 and spread to cover completely. Let stand at cool room temperature until topping is set, about 4 hours.

Using parchment paper as aid, lift brownie sheet from pan and place it on a plate or flat surface. To cut, use a large sharp knife, and wipe the knife with hot moist cloth after each cut. Serve.