Preheat oven and prep pan: Position an oven rack in the middle of the oven and heat to 325°F (162°C). Line the bottom and sides of an 8-inch square baking pan with parchment paper or aluminum foil, leaving an overhang on opposite sides to help remove the baked brownies from the pan.

Melt the butter: Add the butter to a medium saucepan. Place over medium-low heat and cook until the butter melts completely, then turn off the heat. Stay close, and do not let it brown.

Stir in the sugar and cocoa: While the butter is hot, stir in the sugar, cocoa powder, vanilla, and salt. Stir well until blended. Don’t worry if the batter looks gritty.

Cool the mixture slightly: Set the saucepan aside to cool until the mixture is warm, not hot, 5 to 10 minutes.

Add the eggs: Add the cold eggs, one at a time, stirring vigorously after each egg.

Beat in the flour: When the batter looks thick and well blended, add the flour. Use a spoon to beat the flour into the batter until it is very thick and pulls away from the sides of the bowl.

Add optional mix-ins: If using, stir in nuts or chocolate chips (optional).

Bake the brownies: Scrape the thick batter into your prepared pan and spread it evenly. Bake for 20 to 30 minutes, or until the edges look set and the middle is slightly underbaked.

Cool and slice: Let the brownies cool completely in the pan. To cut them, use a sharp chef’s knife, wiping the blade clean after each slice for perfect edges.