How do you prevent a film forming on the top of chocolate brownies when baking them?

To prevent a film from forming on the top of chocolate brownies while baking, ensure you mix the batter thoroughly to incorporate air, which helps create a smoother surface. Additionally, avoid overbaking, as this can lead to a dry top; check for doneness a few minutes before the recommended baking time. Covering the brownies with foil during the first half of baking can also help retain moisture and prevent a crust from forming. Lastly, let them cool in the pan for a bit before cutting, which can help maintain a moist texture.