Preheat oven to 350.
Butter an 8 inch square baking dish and line the pan with parchment paper, leaving a 2 inch overhang on all sides. Place butter and chocolate in a large heatproof bowl set over simmering water. stir frequetly until melted, about 7 minutes. remove from heat and let cool, about 10 to 15 minutes.
Stir sugar into cooled mixture until combined. Whisk in eggs, one at a time. Whisk in vanilla. Gently fold in flour and salt.Pour batter into prepared pan and smooth top with a spatula. Bake until a toothpick comes out clean, about 40-45 min. Run a knife aroung edges of pan. Using parchment paper, lift the brownies out onto a rack. Allow to cool. Transfer to a cutting board cut into squares.