Generously spray a large cookie sheet (jelly roll pan) with cooking spray
Melt butter, baking chocolate and cocoa together in a large heavy saucepan over medium heat and stir until smooth.
Remove from heat.
Gradually whisk in sugar by hand with a wire whisk.
Add eggs, two at a time and stir until well blended after each addition also using a wire whisk, by hand.
Whisk in vanilla.
Combine flour, salt and baking powder in a separate bowl and stir.
Fold flour mixture into chocolate mixture in three additions. Do not over mix, do not use a mixer.
Pour batter into prepared pan. Bake about 35-40 minutes or until toothpick inserted in center comes out with a few moist crumbs attached.
Notes
-Important for this recipe-Do not mix brownies with an electric mixer. This recipe may likely overflow if not mixed by hand in large pan or a large bowl. -The brownie batter fits perfectly into a 12x17x1 jelly roll pan. The batter will almost fill to the top of the pan before cooked. -Laurie tried the brownies both with cake flour and regular flour and didn't notice a significant difference.