10 tablespoons (1 1/4 sticks, 5 ounces or 141 grams) unsalted butter
1 1/4 cups (9 7/8 ounces, 280 grams) sugar
3/4 cup plus 2 tablespoons (2 7/8 ounces, 82 grams) unsweetened cocoa powder (natural or Dutch-process)
1/4 teaspoon salt
1/2 teaspoon pure vanilla extract
2 large eggs, cold
1/2 cup (2 3/8 ounces, 66 grams) 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 foil, leaving an overhang on two opposite sides. Butter the foil.
Combine the butter, sugar, cocoa, and salt in a medium heatproof bowl and set the bowl over a saucepan of barely simmering water. Stir until the butter is melted and the mixture is smooth. Remove the bowl from the pan and set aside briefly until the mixture is warm, not hot.
Stir in the vanilla. Add the eggs one at a time; stir until fully incorporated. Add the flour and stir until you cannot see it any longer, then beat vigorously for 40 strokes.
Bake until a toothpick plunged into the center emerges slightly moist with batter, 25 minutes.
Let cool in pan, then lift up the ends the foil and transfer the brownies to a cutting board. Cut into 16 or 25 squares.