Position a rack on the lower third of the oven. Preheat oven to 325 degrees.

Line the bottom and sides of an 8-inch square baking pan with parchment paper or aluminum foil, leaving an overhang on two opposite sides. (This helps when removing the brownies from the pan, once baked and cooled).

Add enough water to a medium sized saucepan so 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 the bowl over simmering water in the saucpan. If the bottom of the bowl touches the water, remove a little water. Stir mixture occasionally until the butter has melted and the mixture is quite warm. Don’t worry if it looks grainy.

Remove the bowl from heat and set aside for 3 to 5 minutes until warm, not hot. Stir in the vanilla with a spoon. Then add the 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 thick). Beat vigorously here. You want to see the brownie batter pulling away from the sides of the bowl (if you do not have the strength to do this by hand, use a hand mixer). Stir in nuts, if using. Spread evenly into the prepared lined pan.

Bake the brownies at 325 degrees for 20 to 30 minutes or until a toothpick can be inserted into the center and come out with a few moist crumbs. As a visual test, brownies are done when the edges will look dry and the middle still looks slightly underbaked.

Cool completely then remove from pan. For the cleanest lines when cutting, place into the freezer for 20 to 30 minutes to firm up. Cut into 16 squares.