The recipe gives a range of baking times—use the shorter time for metal pans, the longer for Pyrex pans.
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 both chocolates. Remove the pan from the heat; cool slightly. Whisk in the sugar and then the vanilla and salt. The mixture will be somewhat grainy; this is okay. Whisk in the eggs and egg yolk, one at a time, stirring each time until blended. Add the flour, beating until thickened and smooth, 30 to 60 seconds. Pour into the prepared pan and bake until a toothpick inserted in the middle comes out with moist crumbs (not wet batter) clinging to it, 35 to 45 minutes.
Set the pan on a rack until cool enough to handle. Run a paring knife around the inside edge of the pan and then invert the pan onto a flat surface and peel off the parchment. Flip the baked brownie back onto the rack to cool completely. Cut into squares with a sharp knife.
nutrition information (per serving):