+ as smooth as, or smoother than, normal buttercream
+ less sugar, less butter
+ can be made in advance and frozen
What’s not to love?
Best tutorial I’ve found is by Whisk Kid: here.
Leftover egg yolks? Maybe you could make pasta dough. If you want to freeze your yolks, remember to prick them and add a little salt/sugar (depending on whether you plan to use them in cooking/baking) first. Read more about freezing eggs here.