5 ounces semisweet or bittersweet chocolate, chopped
2 ounces unsweetened chocolate, chopped
8 tablespoons (1 stick) unsalted butter, cut into quarters
3 tablespoons cocoa powder
3 large eggs
1¼ cups (8¾ ounces) granulated sugar
2 teaspoons vanilla extract
½ teaspoon salt
1 cup (5 ounces) all-purpose flour
1. Adjust an oven rack to the lower-middle position and heat the oven to 350 degrees. Spray an 8-inch square baking pan with nonstick cooking spray. Fold two 16-inch pieces of foil or parchment paper lengthwise to measure 8 inches wide. Fit 1 sheet in the bottom of the greased pan, pushing it into the corners and up the sides of the pan (overhang will help in removal of baked brownies). Fit the second sheet in the pan in the same manner, perpendicular to the first sheet. Spray the sheets with nonstick cooking spray.
2. In a medium heatproof bowl set over a pan of almost-simmering water, melt the chocolates and butter, stirring occasionally until smooth. Whisk in the cocoa until smooth. Set aside to cool.
3. Whisk together the eggs, sugar, vanilla, and salt in a medium bowl until combined, about 15 seconds. Whisk the warm chocolate mixture into the egg mixture; then stir in the flour with a wooden spoon until just combined. Pour the mixture into the prepared pan, spread into the corners, and level the surface with a rubber spatula. Bake until slightly puffed and a toothpick inserted in the center comes out with a small amount of sticky crumbs clinging to it, 35 to 40 minutes. Cool on a wire rack to room temperature, about 2 hours. Remove the brownies from the pan using the foil or parchment handles and transfer to a cutting board. Cut into squares.