1Heat the oven to 325°F with the rack in the middle. Lightly grease or butter the pan and line it with parchment or foil.
2Melt the chocolate with the butter in a large metal bowl set over a pan of simmering water, stirring occasionally, until smooth. Remove the bowl from the pan and whisk in the sugar.
3Add the eggs, 1 at a time, whisking well after each addition, and whisk the mixture vigorously until it is smooth and cohesive. Whisk in the vanilla and salt. Sift the cocoa over the batter and stir with a whisk until it is incorporated and the batter is smooth. Sift the flour over the batter and stir with the whisk in the same manner until the batter is smooth.
4Spread the batter evenly in the pan and, if desired, sprinkle the top lightly with flakes of Maldon sea salt. Bake 15 minutes.
5Remove the pan from the oven and let the brownies rest 15 minutes at room temperature. Return the brownies to the oven and continue to bake them until a wooden pick comes out clean, 17 to 20 minutes.
6Cool the brownies completely in the pan on a rack. Cut the brownies into serving pieces.