Closely adapted from Alice Medrich. The better of a chocolate you use, the better your results will be. Be sure that the cacao percentage is right around 70%.
Preheat the oven to 350 F. Take an 8-inch square pan and place it in a large section of foil. Lightly mold the foil to the pan, then remove the pan and place the molded foil inside of the pan. Press the foil evenly into the pan and set aside.
Heat a large skillet with an inch or two of water over medium heat. When it is barely simmering, reduce the heat to low and place a large heat-proof bowl inside the skillet.
Add the butter and chocolate to the skillet. Stir frequently with a silicone spatula, scraping the sides and bottom of the bowl, until the butter and chocolate are completely melted and hot to the touch. Stir smooth and remove from the heat.
Stir in the sugar, salt and vanilla. Once again stir smooth.
Add the eggs, one at a time, and stirring each one in immediately.
Scrape the sides and bottom again, and then stir in the flour. Continue to stir briskly until the batter is smooth and cohesive--it should be pulling away from the sides of the pan. This is a case where stirring a little too long is better than under-stirring--make sure the batter is truly coming together.
Scrape the batter into the prepared pan (no greasing necessary). Smooth the top to evenly distribute the batter, but allow some ripples to remain.
Bake for 30-35 minutes (27-33 minutes on 325 F convection), until a toothpick inserted into the center of the pan comes out with only a few moist crumbs. The top will have a thin crust.
Cool completely on a cooling rack, and then use the edges of the foil to lift and remove to a cutting board. Cut into 16 squares.