Preheat to 350°F.
Spray 9" X 9" baking pan with nonstick cooking spray.
Combine 6 ounces dark 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 heat and let mixture cool until lukewarm.
Combine sugar, eggs, vanilla, and salt in large bowl. Whisk in cooled chocolate mixture. Stir in flour, then chopped pecans. Pour batter into prepared baking pan. Bake brownies until tester inserted into center comes out with moist crumbs attached, 25 to 30 minutes. 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 and let stand 5 minutes- or until soft, then whisk until melted and smooth. Pour chocolate mixture over cooled brownies and spread to cover completely. Let stand at cool room temperature until topping is set, approx 4 hours. Cut into 16 squares and serve.