Position rac in the middle of the oven and heat to 350 degrees.
Butter an 8-inch-square pan, line the pan bottom with
parchment and then butter the parchment.
In glass double boiler, melt the butter and both chocolates
together and let cool slightly.
Whisk in the sugar and then the vanilla and salt. The mixture
will be somewhat grainy; this is fine.
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
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.
Serve plain or with vanilla ice cream.16