Preheat oven to 350. Combine butter, sugar, vanilla, and eggs in a bowl. Combine flour, cocoa, baking powder, and salt in a separate bowl. Gradually add dry ingredients to wet ingredients. Pour into a greased 8" square pan. Bake for 40-45 minutes.
I usually bake mine between 35 and 40 minutes so they're chewy in the middle. It makes them less cake-like.