The original recipe didn't use egg yolk and that's what my family prefers. This crumble cake is really CRUMBLY and has a wonderful melt-in-the-mouth texture. If you prefer the cake to hold together a little bit better, add the egg yolk. This results in larger crumbs but the sbrisolona will not be as crumbly. A sbrisolona with egg yolk browns more in the oven.