Hawaiian butterfish is actually a cooking method for seafood in Hawaii, not a separate species of fish. Butterfish, without the "Hawaiian" name attached, is also the common name for a species of fish also known as escolar or walu, which can lead to some confusion.
Hawaiian butterfish is typically made from black cod that is marinated in miso, a Japanese seasoning. It is often found on menus throughout the state under the name "misoyaki butterfish." It is also sold in supermarkets raw but already marinated.
Elsewhere in the world, butterfish refers to a number of different species. As of 2015, escolar has gained popularity on many seafood menus and is sold under the name "butterfish." This same species is known as "walu" in Hawaii and Fiji. Walu is notorious for causing stomach cramps and diarrhea.