Combine flour, cocoa, baking powder and salt in a medium bowl and whisk together, set aside.
Using a mixer, beat together the butter, sugar and vanilla extract in a large bowl until mixed well.
Add in eggs and mix until well combined.
Add the dry ingredients to the egg mixture and mix until well combined.
Pour the batter into the prepared pan and spread evenly.
Bake for 20-25 minutes, or until a toothpick comes out with a few moist crumbs. Set aside to cool.