Pr-heat oven to 350 degrees F, 170 C. Line the bottom and sides of an circle baking pan with parchment paper or aluminum foil. (This helps when removing the baked brownies from the pan, once cooled).
Add cocoa and baking soda to a lage bowl and stir to combine.
Add enough water to a medium saucepan so that it is 1 to 2 inches deep. Heat water until barely simmering. Combine lukewam butter, sugar, cocoa powder in a medium heat-safe bowl. Rest the 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 the 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 the 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 beater or use the wooden spoon or spatula for 40 to 50 strokes. (The batter will be thick).
Add 1\/2 cup of boiling water to bowl and Beat vigorously here. You want to see the brownie batter pulling away from the sides of the bowl (if you do have the strength to do this by hand mixer, use a hand).
Add chopped walnuts to batter and stir until combined.
Pour into prepared baking pan and smooth the top out with a wooden spoon. Bake for between 35 and 45 minutes. Mine took 42 minutes. Test with a toothpick for doneness.
Cool completely then remove from pan. When cool, remove from pan and cut into squares.