You're likely referring to the death cap (Amanita phalloides), which is reportedly quite tasty. But there's also a mushroom that's both deadly poisonous and a sought-after, commercially sold delicacy, the only difference being the method of preparation:

https://en.wikipedia.org/wiki/Gyromitra_esculenta

Although recent research suggests that some poison remains even after careful preparation, and that consumption may even be linked to ALS (Lou Gehrig's disease).