Source, in full
This has been attributed to Hoffman by referencing The New York Times (20 April 1989), but that article (Section A, Page 16) actually says "But the Rabbi also quoted one of Mr. Hoffman's favorite sayings: 'Sacred cows make the tastiest hamburger.'" This statement doesn't attribute the saying to Hoffman, but only says it was a favorite and presumably he used it repeatedly.
Notes from Wikiquote
An earlier version is "Sacred cows make great hamburgers", recorded as an anonymous saying in Encyclopedia of Graffiti (1974) by Robert George Reisner and Lorraine Wechsler. | There are earlier instances of essentially this quote. In the form "Sacred cows make the best hamburger" it appeared in October 1965 in a student newspaper at Pennsylvania State University, The Daily Collegian, saying it was borrowed from Aardvark magazine, according to the Quote Investigator blog.