Preheat oven to 350 degrees. Line a 9" square pan with foil and butter the foil. Sift together flour, cocoa, baking powder, and salt. Melt butter in a 10 - 12 cup saucepan over moderate heat. Stir in sugar and vanilla, and add eggs one at a time. Stir in dry ingredients, and stir in nuts. Spread smoothly in prepared pan. Bake 20 - 25 minutes, or until toothpick inserted in the center just barely comes out clean.
Cool in pan 15 minutes. Cover with rack and turn pan over. Remove pan and foil lining. Cover with a cutting board and turn back over again.
Let stand until cool. Original Recipe : http://www.copykat.com/2009/05/13/hersheys-brownies-why-make-brownies-out-of-a-box-make-home-made-brownies-at-home/#