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Preheat oven to 350°F. Butter a 9-inch square baking pan and line with parchment, leaving a 2-inch overhang on two sides; butter parchment.
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In a medium bowl, whisk together flour, cocoa, and salt. In a large heatproof bowl set over (but not in) a pot of simmering water, combine 1 1/2 cups chocolate (6 ounces) and butter; heat, stirring, until melted. Remove from heat. Immediately add sugar and whisk 10 seconds.
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Add eggs, one at a time, then vanilla; whisk vigorously until glossy and smooth, 45 to 60 seconds. Using a rubber spatula, stir in flour mixture until just combined, then fold in remaining 1/2 cup chocolate (2 ounces). Transfer batter to prepared pan, smoothing top with spatula.
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Bake until top is shiny and crackly and a tester comes out clean, about 35 minutes. Transfer pan to a wire rack and let cool 20 minutes. Using parchment overhang, lift brownies from pan. Let cool completely on rack before cutting into squares. Store in an airtight container at room temperature up to 2 days.
Cook’s Notes
We love the flavor of semisweet chocolate in a brownie, but if you want a more intense taste, you can use bittersweet (about 70 percent)— it won’t change the texture. To add crunch, stir 1/2 cup of walnuts or pecans into the batter.
