To make 2 1/4 cups sour milk, place 7 teaspoons lemon juice or vinegar in a glass measuring cup. Add enough milk to make 2 1/4 cups total liquid; stir. Let stand for 5 minutes before using.
Prepare as directed, except cool pancakes completely on wire racks. Layer pancakes between sheets of waxed paper in an airtight container; cover. Freeze for up to 2 months. To serve, preheat oven to 350°F. Place frozen pancakes on an extra-large baking sheet. Bake about 10 minutes or until heated through. Serve as directed.