Adjust oven rack to middle position and heat oven to 350°F Line 8-inch square pan with foil. Coat lightly with vegetable oil spray. Melt the chocolates, butter, and cocoa in the microwave, stirring often 1 to 3 minutes. Let the mixture cool slightly.
Whisk the sugar, eggs, vanilla and salt together in a large bowl until combined about 15 seconds. Whisk in the melted chocolate mixture until smooth. Stir in the flour until no streaks remain.
Scrape the batter into the prepared pan and smooth the top. Bake until a toothpick inserted into the center comes out with just a few crumbs attached, 35 to 40 minutes.
Let cool completely on a wire rack to room temperature, about 2 hours, before removing the brownies from the pan using the foil and cutting into squares.