Preheat oven to 350°F (175°C). Grease and line a 9×9-inch metal pan with parchment paper, leaving an overhang.
Melt butter in a microwave-safe bowl in 30-second intervals, stirring between each.
Add sugar to the warm butter and whisk vigorously for about 60 seconds.
Add eggs one at a time, whisking after each addition, then whisk in vanilla.
Stir in cocoa powder and salt until smooth.
Gently fold in flour until no dry streaks remain.
Fold in chocolate chips and spread the batter evenly in the prepared pan.
Bake for 30–35 minutes, until a toothpick inserted 1 inch from the edge comes out with moist crumbs.
Cool in the pan for at least 30 minutes, then lift out using the parchment overhang and cool another 15–20 minutes before slicing.
