In this photo illustration, a meatless 'Impossible Slider' sits on a table at a White Castle restaurant, April 12, 2018 in the Queens borough of New York City. The meatless burgers, which sell for $1.99, are about twice the size of White Castle's regular sliders. The patties, made primarily of wheat protein and potato, are the first plant-based burgers sold in an American quick-serve restaurant.

White Castle customers are craving a new kind of burger.

After slinging tiny square beef sliders for decades, the company began testing meat-free burgers at 140 locations in April — and it's become a cult favorite. Now, the Impossible Slider will be available at nearly 400 locations nationwide, White Castle said Wednesday.

The burger is a plant-based patty from Silicon Valley-start-up Impossible Foods that looks, tastes and "bleeds" just like real meat.

The product is made with an ingredient called soy leghemoglobin that releases a protein called heme that gives the meat substitute its distinctive blood-like color and taste. This ingredient was approved by the Food and Drug Administration in July, big win for Impossible Foods, which has been rapidly expanding in the U.S. and abroad.

White Castle's Impossible Sliders are topped with smoked cheddar cheese, pickles and onions and cost $1.99 each.

"Our Cravers definitely developed a hunger for the Impossible Slider," CEO Lisa Ingram, whose family has led White Castle for four generations, said in a statement. "Sales easily exceeded our expectations."

Applebee's, Bareburger and Wahlburgers, among other independent restaurants, are also using the Impossible meat substitute.