Blend oil or melted butter, sugar and vanilla in a mixing bowl.  Add eggs; beat well with spoon.  Combine flour, cocoa, baking powder and salt; gradually add to egg mixture until well blended. If desired, stir in nuts and/or peanut butter chips.  Spread in greased 9-inch square pan.  Bake at 350 degree oven for 20 to 25 minutes or until brownie begins to pull away from edges of pan.  Cool in pan.  Frost if desired; cut into squares.  16 brownies.   Easy and moist.