1 Preheat oven and line baking pan: Preheat oven to 325°F (160°C), with a rack in the lower third of the oven.
Line the bottom of an 8-inch square baking pan with foil or parchment paper in such a way as there is an overhang on two opposite sides to make it easy to lift the brownies out when they’re done.
2 Heat butter, cocoa, sugar, salt in a bowl over simmering water: Place the butter, cocoa, sugar, and salt in a medium-sized metal bowl. Fill a large skillet halfway with water and bring to a bare simmer. Set the bowl of the butter cocoa mixture in skillet of simmering water.
Stir the butter cocoa mixture until the butter has melted and the mixture is smooth and hot to touch.
Remove the bowl from the skillet and let it cool down a bit, from hot to warm.