Position an oven rack in the lower third of the oven and heat to 325°F (163 C). Line the bottom and sides of an 8-inch (20cm) square baking pan with parchment paper or aluminum foil, leaving an overhang on two opposite sides. (This helps when removing the baked brownies from the pan, once cooled).
Add enough water to a medium saucepan so that it is 1 to 2 inches deep. Heat water until barely simmering. Combine butter, sugar, cocoa powder and the salt in a medium heat-safe bowl. Rest bowl over simmering water (if the bottom of the bowl touches the water, remove a little water).
Stir mixture occasionally until the butter has melted and mixture is quite warm. Don’t worry if it looks gritty, it will become smooth once you add the eggs and flour.
Remove the bowl from heat and set aside for 3 to 5 minutes until it is only warm, not hot.

Stir in vanilla with a wooden spoon or spatula. Then, add eggs, one at a time, stirring vigorously after each one.

When the batter looks thick, shiny and well blended, add the flour and stir until fully incorporated, then beat with the wooden spoon or spatula for 40 to 50 strokes. (The batter will be quite thick). Stir in nuts, if using. Spread evenly in lined pan.

Bake 20 to 25 minutes or until a toothpick can be inserted into the center and come out almost clean (you want it to be a little moist with batter). Note: Some have found they need to bake an extra 10 minutes, so keep an eye on the doneness of the brownies and use the toothpick test as your guide.
Cool completely then remove from pan. For the cleanest lines when cutting, place into freezer for 20 to 30 minutes to firm up. Cut into 16 squares.