Preheat oven to 350 degrees. Line an 8-inch square baking pan with parchment paper.
Melt the butter and chocolate in a heatproof bowl over a pot of simmering water, stirring until smooth.
Once the mixture is completely melted, remove from heat. Make sure to wipe the bowl to remove any condensation from the bottom.
Whisk in sugar and eggs one at a time until combined. Also, whisk in cocoa and salt. Lastly, carefully fold flour until there are no streaks or pockets of flour.
Pour batter into pan. Bake until set and a toothpick inserted into the center comes out with moist crumbs, about 35-40 minutes.
Let brownies rest for about 15 minutes before lifting them from the pan using the parchment paper. Remove parchment and transfer to a wire rack. Cut into 9 squares.
Notes
These brownies can be stored at room temperature for up to 3 days.