Don't know if anyone can help me with this question. I have a buttercream recipe that calls for 4 egg whites at room temperature. I do have the eggs but hate wasting the yolk. Can I substitute 100% pure liquid egg whites in the carton? I took a baking class at Sur La Table on layer cakes last weekend and would like to make DS (who will be turning 12 next weekend) a b-day cake with buttercream frosting. Thanks for your help!