These are very popular as a vehicle for smoked salmon and other toppings around Christmas-time. They originate in Russia and were originally made using buckwheat. You can make them ahead of time and add toppings as required.
Whisk the egg whites to soft peaks and fold them gently into the bowl. The idea is to not knock the air out of the egg whites. Cover and leave for 1-2 hours, or overnight if you wish.
Butter for frying the blinis.