Heat the oven to 350°F (175°C). Line an 8-inch square pan with parchment, leaving overhang, and lightly grease the sides.
Melt the unsalted butter in a medium saucepan over low heat, then remove from the heat.
Add the granulated sugar and whisk until glossy and slightly warm, 30–60 seconds.
Whisk in the unsweetened cocoa powder and fine salt until smooth and well combined.
Whisk in the large eggs one at a time until fully incorporated, then whisk in the vanilla extract. Whisk vigorously for 30–60 seconds until the batter looks thick and shiny.
Switch to a spatula and fold in the all-purpose flour just until no dry streaks remain; do not overmix.
Scrape the batter into the prepared pan and smooth the top.
Bake until the edges are set and a toothpick inserted in the center comes out with moist, fudgy crumbs (not wet batter), 20–25 minutes.
Cool the pan on a rack for at least 1 hour to set.
Use the parchment to lift the brownies out, then cut into 12 squares and serve.