Preheat oven to 350 degrees. Line an 8-inch square baking pan with parchment paper. Melt butter and chocolate in a double boiler or a heatproof bowl set over a pot of simmering water, stirring until smooth. Remove from heat, and whisk in sugar. Whisk in eggs, one at a time, until combined. Whisk in cocoa and salt. Fold in flour until combined.

Bake for 30-35 minutes. Let cool in pan on a wire rack about 15 minutes. Lift brownies from pan using parchment. Remove parchment, and transfer to a wire rack. Let cool completely. Cut into 9 squares.