https://www.cnet.com/news/no-april-fools-joke-this-whopper-is-meat-free-and-youd-never-know/

I got a jump on the debut when I arrived at Impossible's Silicon Valley headquarters carrying two bags of Whoppers from the local Burger King. There, J. Michael Melton, Impossible's technical sales and culinary manager, cooked up a batch of the patties they're supplying to Burger King, using the same broiler Burger King uses. He swapped them in for the beef in the Whoppers (with professional dexterity that somehow left the burgers appealing) and I took a couple bites. 

The remarkable thing was how unremarkable they were: Nothing gives away the fact that this Whopper contains a different main ingredient.