Preheat the oven to 325F.

Line a 9 x 9 inch baking pan with parchment paper or foil, leaving some paper hang outside the pan. This will help with removing the fudge once it’s baked.

Melt the butter in a medium heatproof bowl set in a wide skillet of simmering water. Add the sugar, cocoa powder, and salt and heat for about 5-7 minutes, stirring occasionally, until the temperature of the mixture reaches 120-122F – the temperature at which if you insert your finger you would want to immediately pull it out. Remove the bowl from the skillet and set aside for about 5 minutes to cool down, until the mixture is no longer hot to touch.

Using a wooden spoon, stir in the vanilla. Then add the eggs, one at a time, stirring vigorously after each one. Continue stirring until the mixture will look thick, shiny, and well blended. Add the flour and stir until it is well incorporated.

Using the same wooden spoon, beat the mixture vigorously for 50 to 60 strokes. Stir in the nuts, if using.

Transfer the mixture into the line baking pan and spread evenly.

Bake on the lower rack for 20-25 minutes, until a toothpick inserted into the center comes out slightly moist with batter. Remove the brownie from the oven and let completely cool on a cooling rack.

Remove the brownie from the pan by lifting up the ends of the parchment paper or foil. Transfer to a cutting board and cut into 16 pieces. Serve immediately.