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.
Insistently repetitive or continuous: a persistent honking of car horns. Existing or remaining in the same state for an indefinitely long time; enduring: persistent rumors; a persistent infection. Botany Lasting past maturity without falling off, as the calyx on an eggplant or the scales of a pine cone. Zoology Retained permanently, rather than disappearing in an early stage of development: the persistent gills of fishes.