Milk + Lemon Juice (Best Option)
1 cup milk + 1 tablespoon lemon juice (let sit 5-10 minutes)Milk combined with lemon juice creates a mildly acidic environment similar to buttermilk, which activates baking soda to produce carbon dioxide for leavening. The acidity also tenderizes gluten, resulting in a soft crumb.
For best results, use whole milk for moisture and richness, and allow the mixture to sit for at least 5 minutes to curdle properly. Stir gently before adding to the dough.
Compared to original buttermilk, this substitute yields nearly identical texture and flavor, maintaining the balance of tenderness and slight tang that enhances sugar cookies.