5 ounces semisweet or bittersweet chocolate, chopped
2 ounces unsweetened chocolate, chopped
8 tablespoons unsalted butter, cut into quarters
3 tablespoons cocoa powder
3 large eggs
1¼ cups sugar
2 teaspoons vanilla extract
½ teaspoon salt
1 cup all-purpose flour
Adjust oven rack to lower-middle position and heat oven to 350ºF. Line 8-inch square pan with foil, as shown here. Butter the foil.
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, 32-38 minutes. Cool on wire rack to room temperature, about 2 hours, then remove brownies from pan using the foil overhang. Cut into squares and serve. (Do not cut brownies until ready to serve; brownies can be wrapped in plastic and refrigerated up to 5 days.)