Position an oven rack on the middle rung. Heat the oven to 350°F. Butter an 8-inch square pan, line the pan bottom with parchment or waxed paper, and then butter the parchment. In a double boiler over simmering water, melt the butter and chocolate. Remove the pan from the heat, cool slightly. Stir in the sugar, salt, and vanilla. Mix in eggs one at a time, stirring each one in at a time, until blended. Add the flour and cocoa. Beat until the mixture is smooth, usually 30 to 60 seconds. Scrape the batter into the prepared pan and bake until the top is evenly colored with no indentations, usually 35 to 45 min. With a toothpick, insert in the middle of brownies. Toothpick should come out clean, maybe with a few moist crumbs clinging to it, Set the pan on a rack until cool enough to handle. With a paring knife, work around the inside edge of the pan and then invert the pan onto a flat surface. Peel off the parchment. Flip the baked brownies onto a rack. Allow the brownies to cool completely. Cut into squares with a sharp knife. Serve.