Preheat the oven to 350 degrees. Line a 9-by-13-inch pan with aluminum foil or parchment paper and spray with nonstick cooking oil spray Melt the butter in a medium saucepan over medium heat. Remove from the heat and add the cocoa powder, stirring to combine. Let cool slightly. In a large bowl, whisk the eggs together, then add the sugars and vanilla extract, stirring to combine. Add the cooled butter-chocolate mixture, then fold in the flour and salt until just combined; do not over mix. Spread the batter evenly in the pan, making sure the corners are filled. Bake for 40 to 45 minutes or until a crust forms on the surface and a toothpick inserted into the center comes out mostly clean. Transfer the pan to a wire rack; cool completely before cutting the brownies.