What You Need
1 scant cup milk (whole, 2%, or heavy cream)
1 tablespoon lemon juice or white vinegar
Combine the milk and lemon juice. Measure 1 scant cup of milk. Stir in 1 tablespoon of lemon juice or white vinegar.
Let stand 5-10 minutes. Let the mixture stand at room temperature for 5-10 minutes. When it is ready, the milk will be slightly thickened and you will see small curdled bits. (This substitute will not become as thick as regular buttermilk.)
Use the buttermilk. Use this substitute (including curdled bits) as you would buttermilk in your recipe.
Other Buttermilk Substitutes:
Yogurt: Mix 3/4 cup plain yogurt with 1/4 cup water to thin. Use as you would buttermilk.
Sour Cream: Mix 3/4 cup sour cream with 1/4 cup plain water to thin. Use as you would buttermilk.
Cream of Tartar: Mix 1 cup of milk with 1 3/4 teaspoons cream of tarter. Let stand 5-10 minutes until slightly thickened and curdled.