"Extra yolk" refers to an additional egg yolk, which is the yellow part of an egg. It can be used metaphorically to mean having something more than what is required or expected. Full definition
At NestFresh, we get a lot of questions from bakers and dessert - lovers about what to do with extra yolks. (recipechatter.com)
From these necropsies, pathologists map out the probable cause of death by looking at slides from various organs under the microscope, hunting for telltale fused cells that would indicate morbillivirus — akin to sunnyside - up eggs with a bunch of extra yolks. (scientificamerican.com)
Lesson 4 — More than one egg (even an extra yolk) turns the cookies way too cakey and almost rubbery. (paleorunningmomma.com)