Sour cream
Sour cream appears in 2 of the 123 recipes on Burgerman, roughly 2% of the library. In these builds it is doing the melting and the richness, which is usually what separates a good burger from a dry one. You will find it in McDonald's Big Tasty (Copycat) and Seven Layer Sliders, among others. The highest-scoring recipe using it is McDonald's Big Tasty (Copycat) at 9.5 out of 10. It turns up most often alongside tomato, white onion, jalapeno.
