Prepare an 8x4 or 9x5 loaf baking pan with parchment paper, leaving some over the sides to make it easy to remove the brownies from the pan. Preheat the oven to 350°F. Place the butter in a microwave-safe bowl and heat in the microwave until melted (or melt butter over the stove). Add in both sugars and whisk until the mixture pulls away easily from the sides of the bowl as you mix, with a paste-like consistency.
Whisk in the egg and vanilla extract. Add flour, cocoa powder, and salt, then whisk until just combined. Stir in the chocolate chips. Pour the batter into the parchment-lined pan, spreading it around in an even layer and bake for 26-36 minutes.
The top should be crackly and a toothpick inserted in the middle should come out with just a few wet crumbs on it, but not covered in batter or completely clean. The batter should also not be jiggly in the pan. Let the brownies cool completely in the pan before slicing.