Preheat oven to 375.
In a pot on medium heat add butter and melt. Turn heat down and pour in brown sugar and white sugar. Stir until sugar has dissolved. Do not boil. Whisk in cocoa and remove from heat.
To the pot add a pinch of salt, vanilla, and 1 egg at a time, whisking quickly after each addition to avoid cooking the eggs.
Stir in flour and baking powder.
Add a piece of parchment to a 9x9 baking dish. Give a quick spray with cooking spray to ensure all sides have been coated in case like me, your parchment doesn't cover all the way up the sides.
Pour in your batter spreading it around. Give a little shake shake to ensure even distribution.
Place in oven for 30-40 minutes. Test with toothpick to make sure it comes out clean.
Let rest in pan for about 10 minutes and remove to rack. Let cool off completely before serving.