Preheat the oven to 325 degrees. Place the rack in the lower third of the oven. Line the bottom and sides of an 8-inch baking pan with parchment paper, leaving an overhang on two opposite sides.
In a large heat-proof bowl, combine the butter, sugar, cocoa, and salt. Set the bowl in a large skillet of water and simmer over medium-low heat. Stir occasionally until the butter is melted and the mixture is hot to the touch. The mixture will look slightly grainy.
Remove from heat and cool until warm. Add vanilla and stir with a wooden spoon. Add the eggs one at a time, mixing thoroughly after each one. When the batter is well blended, smooth, and shiny, slowly add the flour and stir until it's well combined, then stir vigorously for 30 strokes. Stir in the nuts (if using).
Pour into the baking pan and bake for 20-25 minutes or until a toothpick inserted in the center comes out clean.
Let cool completely, then lift the ends of the parchment paper and transfer them to a cutting board. Cut them into bite-sized pieces. Enjoy.