Milk + lemon juice or vinegar
Closest match1 cup buttermilk = 1 cup milk + 1 tbsp lemon juice or white vinegarThe acid curdles the milk and drops its pH into buttermilk's range, so it still reacts with baking soda for lift and still tenderises gluten for a soft crumb.
MethodStir the acid into the milk and let it stand 5-10 minutes until it thickens slightly and looks curdled before using. Skipping the rest is the most common reason this swap underperforms.
Trade-offThinner than real buttermilk and without its cultured tang, so the finished flavour is a little flatter.