Preheat oven to 350°F. Line an 8x8 baking pan with parchment paper and set aside.
In a large mixing bowl (or using an electric mixer on low), stir together melted butter and dark brown sugar until combined (will be thick).
Add eggs one at a time, stirring well after each addition.
Add vanilla extract and stir to combine.
Add flour, baking soda and baking powder and mix well until combined, but do not over-mix.
Pour batter into the prepared baking pan and bake for 35-40 minutes. Brownies are done when they begin to pull away from the sides and the center is set but still gives slightly when pressed. Brownies will continue to bake from residual heat when removed from oven.