Greek Yogurt (Best Option)
1:1 (1 cup Greek yogurt per 1 cup sour cream)Greek yogurt works well because it has a similar protein and fat content to sour cream, which helps maintain the structure and moisture of brownies. Its natural acidity also reacts with baking soda or powder to aid leavening.
To ensure success, use plain, unsweetened Greek yogurt and avoid varieties with added flavors. If the yogurt is too thick, you can thin it slightly with a teaspoon of milk but keep it thick enough to maintain batter consistency.
Compared to sour cream, Greek yogurt yields brownies with a slightly tangier flavor and equally moist, tender crumb, making it a nearly seamless substitution.