You can make this batter ahead of time and store it in your refrigerator for up to 3 days. The egg whites and skim milk help cut the amount of fat.
Store the batter in an airtight refrigerator container. As a reminder, label with the date it must be used by.
Gently Stirring Batter
Before transferring any of the refrigerated batter into prepared muffin moulds, stir down with a spoon (it will have risen slightly during storage).
Sprinkling with Sugar
Fill each muffin mould two-thirds full. Then, pick up some coarse sugar with your fingers and sprinkle evenly over the tops of the muffins. Move your thumb back and forth to release the sugar.