Combine the butter, sugar, cocoa, and salt in a medium heatproof bowl. Melt over hot water or in the microwave in short bursts. Stir from time to time until the butter is melted and the mixture is smooth. Set aside until the mixture is only warm, not hot.
Stir in the vanilla.. Add the eggs one at a time, stirring vigorously after each one. When the batter looks thick, shiny, and well blended, add the flour and stir until you cannot see it any longer, then beat vigorously for 40 strokes with the wooden spoon or a rubber spatula.
Spread evenly in the lined pan.
Bake until a skewer poked into the center emerges slightly moist with batter, 20 to 25 minutes. Let cool completely on a rack.
Lift up the ends of the baking paper and transfer the brownies to a cutting board. Cut into 16.