For each cup of buttermilk:
1 cup yogurt
1/4 cup milk + 3/4 cup sour cream
1 cup water + 4 tablespoons powdered buttermilk
1/4 cup milk + 3/4 cup yogurt
1 cup milk + 1 tablespoon vinegar
1 cup milk + 1 tablespoon lemon juice
1 cup milk + 1 3/4 teaspoon cream of tartar
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