Mix cocoa in well with the melted butter. Add sugar, vanilla, and eggs, mixing well. Add flour, baking powder, salt, and nuts or chocolate chips. I actually prefer the chocolate chips!! Pour mixture into a greased 9 by 13 inch pan. Bake in a preheated 350F oven for 25 - 30 minutes, until a toothpick inserted comes out almost clean, you do not want to overbake!