The fresher the egg, the thicker the white and better the yolk remains in the center of the white. Older eggs have thinner whites and the yolk frequently slides to the edge of the white.
When working with phyllo dough, keep it covered with a damp cloth so it does not dry out and crack. Immediately place any remaining dough in a resealable plastic freezer bag and return it to the freezer.
Assemble the buttered phyllo pastry squares in the baking cups several hours ahead and let stand at room temperature, covered with a damp cloth and plastic wrap. Add eggs and bake shortly before serving.