In a medium-sized mixing bowl, combine the melted butter, sugar, brown sugar, salt, and cocoa powder. Mix until combined, then add the eggs and vanilla and whisk to combine.
Prepare your baking dish by spraying with non-stick cooking spray and then lining it with parchment paper. Line the bottom of 9" x 9" baking dish for thick brownies or 9" by 13' baking dish for thin brownies.
Bake in a preheated oven for 30-40 minutes for a 9" x 13" pan or about 60 minutes for a 9" x 9" pan. Bake until the edges of the brownies are crisp and the center is just set.