Heat oven to 325 F. Spray a 9x9 baking pan with cooking spray.
In a medium saucepan, add the butter over medium heat. Tip: Using a light-colored pan will make it easier to tell when the butter has browned.
Swirl the pan occasionally, but try not to stir. As the butter melts, it will foam and the color will turn from lemony-yellow to an amber brown. This takes anywhere between 5-10 minutes. Just be sure to watch carefully, as you do not want it to burn.
Once the butter is amber colored, immediately remove from heat. Let cool slightly for 10-15 minutes.
Once slightly cooled, add sugar and stir well to combine. Note: you can either do this in the pan you melted the butter in or, if it’s too small, transfer butter to a large bowl.
Add cocoa powder and stir until well combined.
Add eggs, one at a time, stirring until well combined and the batter is shiny.
Add vanilla and stir to combine.
Add flour and salt, stirring until combined, and then give the batter 60 good strokes with your spatula.
Fold in dark chocolate chips.
Transfer batter to prepared pan.
Bake for 28-32 minutes for 9x9 pan (add another 5-8 minutes baking time for an 8x8 pan), until the edges are crackly and the middle is set but slightly jiggly. A toothpick should come out with just a few crumbs on it.
Cool, slice and enjoy!