Buttermilk (Best Option)
1 cup buttermilk per 1 cup whole milkButtermilk works well because it has a similar fat content and moisture level as whole milk but adds acidity, which reacts with baking soda to improve rise and crumb structure. The proteins and fats help maintain moisture and richness.
When using buttermilk, ensure your recipe has baking soda to balance the acidity; if not, add about 1/4 teaspoon baking soda per cup of buttermilk. Watch for a slightly tangier flavor, which complements banana bread.
The final bread will be moist with a tender crumb and a subtle tang that enhances the banana flavor, often considered an improvement over whole milk.