Preheat oven to 350 degrees. Grease and flour a 13x9x2 inch pan. Sift flour, cocoa, baking powder, and salt onto a sheet of waxed paper. In a large mixing bowl, cream butter, shortening and sugar with mixer for 1 minute. Add eggs and continuing mixing until well blended, about 2 minutes. Scrape sides if necessary. Turn to low speed on mixer and gradually add flour mixture. When ingredients are moistened add vanilla extract. Turn mixer on a medium speed and keep mixing for 1 more minute. Scrape sides if necessary. Turn off mixer. Stir in nuts (optional). Pour into prepared pan and bake for 25 to 30 minutes. Cool in pan or wire rack.