Oven 350 deg F
line 8 inch square baking dish with parchment paper leaving a slight overhang on sides.
Melt chocolate and butter in double boiler. 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.
Pour batter into pan. Bake until set and a toothpick comes out with moist crumbs, 35 - 40 minutes. Let cool slightly in pan (15 minutes), lift out with parchment paper and cut into squares.