Pour the tablespoon of vinegar into a 1 cup measurer. Fill the rest of the 1 cup measurer with milk.
Gently stir and let the mixture sit for about 5 minutes. This will allow the milk to slightly curdle, giving it a buttermilk-like texture and taste.
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Notes
More buttermilk substitute options:
1 cup buttermilk = 1 Tablespoon white vinegar + enough milk to measure 1 cup
1 cup buttermilk = 1 Tablespoon lemon juice + enough milk to measure 1 cup
1 cup buttermilk = 1 cup plain yogurt
1 cup buttermilk = 1 and 3/4 teaspoon cream of tartar + 1 cup of milk
Dairy-Free and Vegan: Instead of dairy milk, use coconut milk or soy milk! Just add 1 Tbsp vinegar or lemon juice to a 1 cup measuring cup, then fill the rest of the way with coconut or soy milk. Stir and let sit for 5 minutes then use in recipe!How to Freeze Buttermilk: Buttermilk freezes great for up to 3 months. You could even pour in an ice cube tray so it's easy to use small amounts as needed.