Adjust oven rack to lower-middle position and heat oven to 350°F Spray 8-inch square baking pan with nonstick vegetable cooking spray.
Fold two 12-inch pieces of foil lengthwise so that they measure 7 inches wide. Fit one sheet in bottom of greased pan, pushing it into corners and up sides of pan; overhang will help in removal of baked brownies. Following illustration 1 below, fit second sheet in pan in same manner, perpendicular to first sheet. Spray foil with nonstick cooking spray.
In medium heatproof bowl set over a pan of almost-simmering water, melt chocolates and butter, stirring occasionally until mixture is smooth. Whisk in cocoa until smooth. Set aside to cool slightly.
Whisk together eggs, sugar, vanilla, and salt in medium bowl until combined, about 15 seconds. Whisk warm chocolate mixture into egg mixture; then stir in flour with wooden spoon until just combined. Pour mixture into prepared pan, spread into corners, and level surface with rubber spatula; bake until slightly puffed and toothpick inserted in center comes out with a small amount of sticky crumbs clinging to it, 35 to 40 minutes.
Cool on wire rack to room temperature, about 2 hours, then remove brownies from pan using foil handles (see illustration 2, below).
Cut into 1-inch squares and serve. (Do not cut brownies until ready to serve; brownies can be wrapped in plastic and refrigerated up to 5 days.).