The butter not only adds richness to the burger and prevents it from drying out, but it also butters the bun for you. We're not going to downplay it, these burgers are decadent. But, if you're going to eat a burger, you may as well do it right. And, this recipe does it so right.
The word persistence means the quality of being determined to do or achieve something. It's tenacity. It means firmness of purpose. It is a continuing, repeating or lasting of something. Use the link and read a few similar definitions.
Definition of persistent for English Language Learners. : continuing to do something or to try to do something even though it is difficult or other people want you to stop.