Moist and cake-like, these brownies are sometimes called Texas sheet cake. Because the bars are baked in a 15x10-inch pan, you'll have enough to feed a crowd.
To make 1 cup sour milk, place 1 tablespoon lemon juice or vinegar in a glass measuring cup. Add enough milk to make 1 cup total liquid; stir. Let mixture stand for 5 minutes before using. Use 1/2 cup sour milk in the brownies and 3 tablespoons sour milk in the frosting. Discard remaining sour milk. Or reconstitute dry buttermilk by following the directions on the powdered buttermilk container.