10 tablespoons unsalted butter
1 1/4 cups sugar
3/4 cup plus 2 tablespoons unsweetened cocoa powder (natural or Dutch-process)
1/4 teaspoon salt
1/2 teaspoon pure vanilla extract
2 large eggs, cold
1/2 cup all-purpose flour
Position a rack in the lower third of the oven and preheat the oven to 325?F. Line the bottom and sides of an 8?8-inch square baking pan with parchment paper or foil, leaving an overhang on two opposite sides.
Combine the butter, sugar, cocoa, and salt in a medium heatproof bowl and set the bowl in a wide skillet of barely simmering water. Stir from time to time until the butter is melted and the mixture is smooth and hot enough that you want to remove your finger fairly quickly after dipping it in to test. Remove the bowl from the skillet and set aside briefly until the mixture is only warm, not hot. It looks fairly gritty at this point, but don?t fret ? it smooths out once the eggs and flour are added.
Stir in the vanilla with a wooden spoon. 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 toothpick plunged into the center emerges slightly moist with batter, 20 to 25 minutes. Let cool completely on a rack. (Deb suggests putting the cooled pan in the fridge or freezer for a while, which helps if you want clean lines when cutting the brownies.)
Lift up the ends of the parchment or foil liner, and transfer the brownies to a cutting board. Cut into 16 or 25 squares.