Preheat oven to 350 degrees. Line an 8-inch square baking pan with parchment, leaving a slight overhang on all sides. 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, 1 at a time, until combined. Whisk in cocoa and salt. Fold in flour until combined.
Pour batter into pan. Bake until set and a toothpick inserted into the center comes out with moist crumbs, 35 to 40 minutes. Let cool slightly in pan, about 15 minutes. Lift brownies from pan using parchment. Remove parchment, and transfer to a wire rack. Let cool completely. Cut into 9 squares.
To add crunch to the brownies, stir 1/2 cup of walnuts or pecans into the batter.