Combine dry milk, flour and salt; mix well. Cut in butter until mixture resembles fine crumbs. Store in an airtight container in the refrigerator for up to 3 months.
TO PREPARE WHITE SAUCE: For thin white sauce, combine 1/3 cup mix and 1 cup water in saucepan. For medium white sauce, use 1/2 cup mix, for thick white sauce, use 3/4 cup of mix. Bring to a boil over medium heat; cook and stir for 2 minutes. Yield: 1 cup per batch.