Adjust oven rack to medium position and preheat oven to 350°F. Spray 8- by 8-inch baking pan with non-stick pan spray. In heatproof bowl over pan of barely simmering water, heat chocolate and butter until just melted, stirring occasionally. Remove from heat and let cool slightly.
Sift flour and cocoa into medium bowl; set aside. In large bowl, whisk sugar with eggs, yolk, salt, and vanilla until light, about 30 seconds. whisk in chocolate mixture. Using wooden spoon, stir in chocolate mixture until combined. Stir in flour mixture until combined.
Pour batter into prepared pan and smooth top. Evenly sprinkle walnuts over top. Bake until just set and toothpick inserted into center comes out with moist crumbs about 30 minutes. Let cool on wire rack about 45 minutes before cutting into squares.