Reattach the clean and dry bowl to the mixer. Using the whisk attachment beat the reserved egg whites plus two more egg whites for a total of 8 egg whites for 1 minute until foamy. Add the cream of tartar and turn on high. Beat the whites until a
stiff peak forms. Fold the whites into the batter in three batches using a rubber spatula, being careful to keep as much air in the mixture as possible.