Set the oven rack to the middle position. Preheat to 350ºF (177ºC).
In an 8 by 8-inch square pan, place two 8-inch wide pieces of foil in the pan perpendicular to each other to create a sling. Press down on the sides and corners to ensure that the foil is flush with the pan. There should be some overhang on the sides.
Grease the foil with cooking spray.
In a small bowl, heat the bittersweet and unsweetened chocolate for 2 minutes in the microwave on 50% power. This will just begin to melt the chocolate.
Add slices of butter to the chocolate, stir to roughly combine.
Microwave at 50% power in 30-seconds increments, stirring after each duration until the mixture is completely melted and combines. About 1 ½ to 2 minutes.
Whisk the cocoa powder into the chocolate mixture until smooth.
Allow the chocolate mixture to cool to 95ºF (35ºC) and below to prevent the eggs from curdling.
In a large bowl whisk together the sugar, eggs, vanilla, and salt until combined and smooth.
Whisk in the cooled chocolate mixture into the sugar mixture.
Gently fold the flour into the sugar mixture until no flour pockets remain, do not overmix the batter.
Evenly spread the brownie batter into the greased baking pan.
Bake until a toothpick inserted into the center comes out with just a few crumbs attached, 30 to 35 minutes. Rotate the pan halfway through baking.
Completely cool the brownies inside the pan placed on a wire rack, about 1 ½ to 2 hours.
Use the foil sling to carefully lift and remove the brownie from the tray. Discard the foil and cut 16 individual pieces.
Semi-sweet or dark chocolate chips can be substituted for the bittersweet bar.
Sprinkle brownies with a little flaked sea salt before and after baking to enhance the chocolate flavor.
For gooier, wet brownies make sure to remove them promptly from the oven at 28 minutes, and test for doneness. Bake longer as needed, checking every minute. The longer the brownies bake, the cakeier they will become, which can happen quickly.
Make it Gluten-Free: Replace all purpose flour with gluten-free flour. I reccomend Bob's Red Mill gluten-free 1:1 baking flour.
Recipe adapted from America's Test Kitchen, The Perfect Cookie.