1. Preheat oven to 350 degrees. Butter an 8-inch square baking pan or heatproof glass dish. Line pan with parchment paper, leaving a 1-inch overhang on 2 sides. Set aside. Place butter and chocolate in a large heatproof bowl set over (but not touching) simmering water; stir frequently until chocolate and butter are melted, about 7 minutes. Remove bowl from heat; let cool to room temperature, 10 to 15 minutes.

2. Stir sugar into cooled chocolate mixture until combined. Whisk in eggs one at a time, whisking until smooth after each addition. Whisk in vanilla. Gently fold in flour and salt.

3. Pour batter into prepared pan, and smooth top with an offset spatula. Bake until cake tester inserted in the center comes out with only a few moist crumbs attached, 40 to 45 minutes. Transfer pan to a wire rack to cool completely.

4. Run a knife or offset spatula around the edges of the pan. Using parchment, lift brownies out of pan and onto the rack. Transfer to a cutting board; cut into 2-inch squares.