One word: Yum. These were super easy, and we demolished these in a weekend, not even. Whoops. And…that’s all I have to say about that!
With the oven rack in the lower-middle position, preheat the oven to 350 degrees. Fold two 16-inch pieces of foil or parchment paper lengthwise to measure 8 inches wide. Fit the foil into an 8 x 8 square baking pan, pushing it into the corners and up the sides of the pan. The overhang will give you something to pull up on to be able to remove the brownies. Fit a second sheet in the pan folded in the same manner, perpendicular to the first sheet. Spray the sheets with non-stick cooking spray.
In a medium heatproof bowl set over a pan of almost-simmering water or in a microwave on 50% power in 30 second increments, stirring EVERY 15-30 seconds, melt the chocolates and butter together. If using the stovetop, stir occasionally until smooth. Once melted together, whisk in the cocoa until smooth, and then set aside to cool.
In a medium bowl, whisk together eggs, sugar, vanilla, and salt until combined, about 15 seconds. Once combined, whisk the warm chocolate mixture into the egg mixture. Finally stir in the flour with a wooden spoon until just combined. Pour the mixture into the prepared pan, spread into the corners, and level the surface with a rubber spatula. Bake until slightly puffed and a toothpick inserted in the center comes out with a small amount of sticky crumbs clinging to it, 35 to 40 minutes. Cool on a wire rack to room temperature, about 2 hours. Remove the brownies from the pan using the foil or parchment handles and transfer to a cutting board to cut into squares.