1- Preheat oven to 325°F (162°C) and position a rack in the middle. Line an 8-inch square baking pan with parchment paper or foil with an overhang.

2- Melt butter in a medium saucepan over medium-low heat without browning it, then turn off heat.

3- Stir in sugar, cocoa powder, vanilla extract, and salt while butter is still hot, until blended. The batter may look gritty at this stage.

4- Let the mixture cool for 5 to 10 minutes until warm but not hot.

5- Add cold eggs one at a time, stirring vigorously after each addition.

6- Beat in flour with a spoon until the batter is thick, glossy, and pulls away from the sides (about 40 to 50 strokes).

7- Stir in optional nuts or chocolate chips if desired.

8- Spread batter evenly in the prepared pan and bake for 20 to 30 minutes, until edges are set and center is slightly underbaked. A toothpick should come out with some moist batter.

9- Cool brownies completely in the pan. For cleaner slices, chill in the fridge for 1 to 2 hours before cutting. Use a sharp knife, wiping clean between cuts.