Preheat the oven to 325°F (163ºC). Grease a 9-inch square baking pan generously and line with parchment paper (see notes for tips).
In a large mixing bowl, combine the brownie mix, egg, and sweetened condensed milk with a spatula, scraping the sides and the bottom of the bowl as needed. Stir until fully combined and thick. It will take a few minutes to come together.
Fold in the semisweet chocolate chips.
Spread the batter evenly in the prepared pan.
Bake for 35–40 minutes, or until the center is set and a toothpick inserted in the middle comes out with a few moist crumbs. The edges should look set and start pulling away from the pan, and the top will start to crack.
Cool completely (30 minutes to 1 hour) in the pan before cutting into squares.
Notes
Note on servings: The serving count is based on the box's recommended 18 servings per box. In my photos, I cut it into 9 large brownies.Tips for lining your pan: To line your pan, start by tearing off a sheet of parchment paper long enough to cover the bottom of the pan with a few inches of overhang on two sides. Lightly grease the pan with butter or nonstick spray so the parchment sticks in place, then press the paper into the bottom and up the sides, letting the extra parchment hang over the edges to create easy “handles” for lifting the brownies out later. If you’d like to cover all four sides, place a second sheet in the opposite direction.