Preheat oven to 325°F (160°C) and line an 8-inch square baking pan with parchment paper.

Combine the butter, sugar, cocoa powder, and salt in a medium heatproof bowl.

Set the bowl over a saucepan of simmering water and stir until the butter is melted and the mixture is smooth and hot to the touch.

Remove the bowl from the heat and let it cool slightly until warm.

Stir in the vanilla extract using a wooden spoon or spatula.

Add the eggs one at a time, stirring vigorously after each addition until the batter looks shiny and well-combined.

Add the flour and stir until it disappears, then beat vigorously for 40 strokes with a wooden spoon to develop structure.

Spread the batter evenly in the prepared pan.

Bake for 20 to 25 minutes, or until a toothpick inserted into the center comes out slightly moist with a few crumbs.

Let the brownies cool completely in the pan on a wire rack before cutting into 16 squares.