Position your oven rack in the lower third of your oven.
Preheat oven to 325ºF
Line a 8-inch square pan with parchment or alumni paper. Spray lightly with non-stick cooking spray.
Combine butter, sugar, cocoa powder and salt in a large bowl, preferably glass, and place bowl over a pan of water that is just barely simmering. Stir occasionally until butter is melted and mixture is combined.
Remove from heat and allow to cool for 5-10 minutes. You do not want the mixture too hot when you add the eggs.
After mixture has cooled slightly, add vanilla and mix well.
Add eggs, one at a time, and beat vigorously until very well mixed.
In a separate bowl combine flour and baking powder, and stir to combine.
Add flour mixture to batter, and mix very well to combine. While we are often told not to over mix brownies, this batter needs to be stirred vigorously. Stirring about 40 strokes with a spoon is a good guide.
If adding chocolate chips and or nuts, fold them into the batter.
Bake for 30 to 35 minutes, or until a tester comes out with some moist crumbs attached. Allow to cool on a wire rack.
Serve and enjoy. If not eating the same day, wrap or cover tightly.
Recipe lightly adapted from Alice Medrich's Best Cocoa Brownie Recipe.