Add a little butter. Recently tried Virginia Willis' Deviled Eggs recipe. So good! She recommends pushing the yolks through a sieve or using a food processor. I only used 4 eggs to test the recipe and didn't want to dirty extra dishes. So I just mashed the yolk with the room-temp. butter before mixing in the other ingredients, thinking the cold mayo and dijon might cause the butter to clump. Worked like a charm. Silky smooth yolks.