Position rack in center of oven and preheat to 350°F (180°C). Generously butter 8 x 8 x 2-inch square metal baking pan; dust with flour.
Mix first 3 ingredients in small bowl. Stir chocolate and butter in top of double boiler set over simmering water until melted and smooth. Turn off heat. Let chocolate stand over water.
Using electric mixer, beat eggs and sugar in large bowl until mixture thickens and falls in soft ribbon when beaters are lifted, about 5 minutes. Beat in vanilla. Stir in flour mixture in 2 additions, blending well after each. Gradually add warm chocolate to egg mixture, beating until just combined. Stir in walnuts.
Pour batter into prepared pan. Bake brownies in preheated oven until top is set and tester inserted into center comes out with moist crumbs attached, about 35 minutes. Cool completely in pan on rack.
Whisk all ingredients in small saucepan over medium-low heat until melted and smooth. Pour evenly over brownies in pan.
Chill brownies until ganache is set, about 2 hours. Cut into 16 squares. (Can be prepared 2 days ahead. Cover; chill. Serve at room temperature.)
Makes 16 brownies.
Source: Bon Appétit Magazine, February 2001
Date: May 6, 2004