Preheat the oven to 325F/160C. Line an 8×8-inch baking pan with parchment paper.

In a large heat-proof bowl, combine the butter, sugar, cocoa and salt, and melt in the microwave in 30 second-intervals, stirring in between each interval, until the butter is melted and the mixture is smooth. (Or you can set it over a saucepan of simmering water, stirring occasionally).

The mixture will appear gritty and that is ok. If the mixture is hot, set aside for a few minutes until it’s warm.

Using a wooden spoon or a rubber spatula, stir in vanilla extract. Add the eggs, one at a time, stirring after each one. Stir until the batter is thick and shiny. Add flour and stir until the mixture is mixed, then beat vigorously for 40 strokes. The batter will be very thick. Stir in the nuts, if using. Spread evenly in pan and smooth the top.

Bake until a toothpick inserted into the center of the brownies comes out with a few moist crumbs, 25-30 minutes. Let cool completely on a rack before cutting into squares (refrigerate them first for a clean, flawless cut with clean lines).