Stir 1 tablespoon of lemon juice or white vinegar into a measuring cup, add milk to the 1-cup line (about 227 g total), and rest 5–10 minutes until slightly thickened.
Buttermilk needed
Lemon juice / vinegar
Milk
½ cup
1½ tsp
to ½ cup
1 cup
1 tbsp
to 1 cup
1½ cups
1½ tbsp
to 1½ cups
2 cups
2 tbsp
to 2 cups
Alternatives: ¾ cup plain yogurt + ¼ cup milk, or plain kefir 1:1. See buttermilk in grams.