5 ounces bittersweet or semisweet chocolate, chopped
2 ounces unsweetened chocolate, chopped
8 tablespoons unsalted butter, cut into 4 pieces
3 tablespoons cocoa
3 large eggs
1-1/4 cups (8-3/4 ounces) sugar
2 teaspoons vanilla extract
1/2 teaspoon salt
1 cup (5 ounces) all-purpose flour
[Editor's note: the original postings omitted the eggs by mistake]
1. Adjust oven rack to lower-middle position and heat oven to 350 degrees F.
2. Spray 8-inch square baking pan with vegetable oil spray. Make foil sling by folding 2 long sheets of aluminum foil so that they are as wide as the pan. Lay sheets of foil in pan perpendicular to one another, with extra foil hanging over edges of pan. Push foil into corners and up sides of pan, smoothing foil flush to pan. Spray foil with vegetable oil spray.
3. In medium heatproof bowl set over 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.
4. 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 small amount of sticky crumbs clinging to it, 35 to 40 minutes.
5. Let cool on wire rack to room temperature, about 2 hours, then remove brownies from pan using foil handles.
6. Cut into 1-inch squares and serve. Do not cut brownies until ready to serve; brownies can be wrapped in plastic and refrigerated for up to five days.