Heat the oven to 350°F (175°C). Line the pan with parchment, leaving handles above two opposite sides, and lightly grease the paper.
Melt butter and chopped chocolate together in a microwave-safe bowl in 20-second bursts, stirring between each. Let cool for 5 minutes.
Whisk in sugar, then eggs and vanilla. Whisk by hand for about 1 minute, until the mixture looks glossy and slightly thickened.
Sift cocoa over the bowl, then add flour and salt. Fold gently with a spatula until no dry pockets remain.
Spread batter evenly into the prepared pan, pushing it into the corners. Tap the pan once gently to release large trapped air pockets.
Bake 25–30 minutes, until edges are set and a tester near the center shows moist crumbs, without liquid batter.
Cool completely in the pan on a rack, about 90 minutes. The center firms as it cools, so avoid cutting while hot.
Lift out using the parchment handles and cut into 16 squares. Wipe the knife between cuts to keep the fudgy edges neat.
