20 tablespoons (10 ounces or 280 grams) unsalted butter
2 1/2 cups (1 pound, 3 3/4 ounces/560 grams) sugar
1 3/4 cup (5 3/4 ounces/165 grams) unsweetened cocoa powder (I used Valrhona)
1/2 teaspoon salt
1 teaspoon pure vanilla extract
4 large eggs, cold
1 cup (130 grams/4 3/4 ounces) all-purpose flour
Position a rack in the lower third of the oven and preheat the oven to 325°F. Line a 13x9" (I used a quarter sheet pan and it was perfect) baking pan with parchment paper or foil, leaving an overhang.
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 occasionally 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. Remove the bowl from the skillet and set aside until the mixture is only warm, not hot. It will look fairly dodgy but it will even out once the rest of the ingredients are added.
Stir in the vanilla, then 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 a wooden spoon or rubber spatula. Spread evenly in the lined pan.
Bake until a toothpick plunged into the center emerges slightly moist with batter, about 25 minutes. Let cool completely on a rack. (Deb refrigerates the brownies to ensure clean cuts.)
Lift up the ends of the parchment or foil liner, and transfer the brownies to a cutting board. Cut into squares.