Vanillekipferl are shortbread cookies, meaning they're made with a high butter content and generally without the addition of egg. You'll find some recipes that call for eggs but traditionally they are not included. Adding an egg yolk or two does make the dough easier to work with, especially for less experienced bakers, but the texture is far superior without the egg; it's lighter, looser, and more delicately crumbly with an almost melt-in-your-mouth texture.