Preparation

• Position a rack in the center of the oven and heat the oven to 325°F.Line the bottom and sidesof a 9×13-inch straight-sided metal baking pan with heavy-duty aluminum foil, leaving about a 2-inch overhang on the short sides. Lightly butter the foil.

  Put the butter in a large (4-quart) saucepan over medium-low heat and stir occasionally until melted, about 2 minutes. Off the heat, whisk in the cocoa powder until smooth, 1 minute. Add the sugar and salt, and whisk until well blended. Use your fingertip to check the temperature of the batter—it should be warm, not hot. If it’s hot, set the pan aside for a minute or two before continuing.

  Whisk in the eggs, two and then three at a time, until just blended. Whisk in the vanilla until the batter is well blended. Sprinkle the flour over the batter and stir with a rubber spatula until just blended.

  Scrape the batter into the prepared pan and spread evenly. Bake until a toothpick inserted in the center comes out with small bits of brownie sticking to it, 35 to 45 minutes. For fudgy brownies, do not overbake. Cool the brownies completely in the pan on a rack, about 3 hours.

  When the brownies are cool, use the foil overhang to lift them from the pan. Invert onto a cutting board and carefully peel away the foil. Flip again and cut into 24 squares. Serve immediately or wrap in plastic and store at room temperature for up to 3 days. They can also be frozen in an airtight container or freezer bag for up to 1 month.

Embellish the brownies with one or more of these variations if you like:

Add-Ins:Stir 1 cup chopped toasted nuts (such as pecans, unsalted peanuts, or walnuts) or 3/4 cup baking morsels (peanut butter, or bittersweet, semisweet, or white chocolate) into the batter after adding vanilla.

Chocolate Glaze and Toppings:Put 6 oz. chopped bittersweet chocolate and 4 oz. unsalted butter, cut into 4 pieces, in a small, heatproof bowl set over a small pot of barely simmering water and stir frequently until melted, about 3 minutes.(Alternatively, the chocolate and butter can be melted in a microwave.) Pour the glaze onto the center of the cooled brownie (after unmolding but before cutting into squares). Using a small offset spatula, spread the glaze evenly over the brownie. If you like, sprinkle your choice of topping (crushed toffee or peppermint candies, or chopped nuts) over the just-poured glaze. Let the glaze cool until set, 1-1/2 to 2 hours, then cut and serve.