McDonald’s is tweaking the way it makes its signature burgers in about a dozen large cities and plans to roll out the changes nationwide. The changes are subtle, but McDonald’s said Monday they’ve resulted in the company’s “best burgers ever.” The changes start with new buns, that McDonald’s said are “softer” and “pillowy.” They are also using more Big Mac sauce and are going to start adding onion to the patties while they’re on the grill. The recent improvements also feature “perfectly melted cheese. The company’s director of culinary innovation says, “We found that small changes, like tweaking our process to get hotter, meltier cheese and adjusting our grill settings for a better sear, added up to a big difference in making our burgers more flavorful than ever.” The changes are being applied to the signature Big Mac, the McDouble, and the classic cheeseburgers and hamburgers. The fast food chain said it plans to implement the changes across the U.S. by next year.