Preheat the oven to 350 degrees. Butter a 9 X 13 inch pan.
In a mixing bowl cream together the butter and sugar until fluffy. Add the eggs one at a time, beating after each addition. Add the vanilla.
In a medium-sized bowl whisk together the cocoa powder, flour, baking powder and salt. Add to the butter mixture and stir to combine.
Spread the batter into the prepared pan. Bake for 25-30 minutes or until just set in the center. Cool completely and cut into bars.