McDonald's added butter to one of its most popular menu items - and sales are soaring

McDonald's made a minor change to its Egg McMuffins, and sales of the sandwiches have exploded as a result.

The company recently changed its recipe for the sandwiches to replace margarine with butter.

Adding butter had an immediate impact, driving up sales of Egg McMuffins by double digits, executives said on an earnings call Thursday.

In addition to the McMuffins, executives said another butter-based recipe also helped drive McDonald's sales in the most recent quarter: the buttermilk crispy sandwich.

"It outperformed the high end of our expectations," McDonald's CEO Steve Easterbrook said of the sandwich.

The company on Thursday reported a 0.9% increase in US same-store sales in the most recent quarter, following seven straight quarters of declines.

McDonald's has also been tweaking how it cooks and prepares its burgers.

The fast-food chain has increased the size of its quarter-pounder beef patties from 4 ounces when raw to 4.25 ounces, according to the company's website.

The 4-ounce patties shrink to 2.8 ounces after cooking.

It also began toasting its buns for an extra five seconds, making them 15 degrees warmer, and changing how the beef patties are seared and grilled in order to make them juicer.

The company hasn't said how those changes have affected burger sales.