What do you do with the yolk when a recipe calls for whipped egg whites

I make Sara Moulton's yellow cake: https://livelovelaughlearn.wordpress.com/2010/09/27/classic-white-cake/
There are instructions on the bottom to make a yellow cake.

I usually just make a custard using the 1-2-3 method. 1 cup sugar, 2 cups milk, 3 egg yolks (With a bit of corstarch to firm it up and a teaspoon or two of vanilla extract to taste)

If it's just one or two yolks, and you just happen to be making meatballs, meatloaf or fishballs for dinner, add them to the mixture; I find that as a binder, they make for a more tender outcome than whole eggs (learned this years ago from a friend's Russian babysitter).

I freeze then in a container and keep track of new yolks added by having a tally on the lid then use them to make curd. Lemon, lime, passionfruit, raspberry... I use a nigella lawson recipe.

zabaglione

Creme brûlée

save to make lemon curd.

Scramble them and feed them to my puppies as a treat.

If you save enough yolk, you can make http://eatapieceofcake.blogspot.com/2014/01/kue-lapis-surabaya.html which needs 30 egg yolks.

Use them in ice cream, semifreddo (there's recipes that use just whites and whole eggs), french parfait, zabaglione, or french buttercream. On the savory end, try mayo/aioli, pasta carbonara, mix into to make extra rich frittatas...

If you're confident in your pastry skills, random egg yolks can frequently be incorporated with good effect. Don't be afraid to improvise when baking- well, maybe a little afraid; you can screw up pretty bad, but there's more leeway than a lot of people believe and you're not a bakery; why would you want everything to come out exactly the same all the time?

Same as MMH. Save in fridge a few days or freezer up to a year.
Uses include mayo, lemon curd, pie filling, sauces.

I save em and mix em in with my scrambled eggs.