To “butterfly” a food (such as a steak) is to place it flat, cut it nearly in half starting from one side almost completely to the other, and to then open and spread the food out, as if like a butterfly’s wings. This allows a thick food to cook more quickly and evenly. The name “butterfly cut” is used for this procedure. A “butterfly steak,” of course, contains no butterflies.
 
“A California specialty worth trying at home is called a ‘butterfly steak’” was cited in print in 1951. “The Pump Room’s famous butterfly steak” (the Pump Room at the Ambassador East Hotel in Chicago, IL) was cited in print in 1955.
 
   
Google News Archive
29 October 1942, The News and Courier (Charleston, SC), pg. 10, col. 7 ad:
$2.00 Broiled Boneless Sirloin, (Butterfly Cut) Mushrooms.
(Henry’s—ed.)
 
Google Books
Book for Men ...
By Bert Bacharach
New York, NY: Barnes
1953
Pg. 126:
A California specialty worth trying at home is called a “butterfly steak.” It’s a minute steak made even thinner by slitting almost all the way through before cooking. Next time you cook a thick filet mignon, slice it from top to bottom (but not quite all the way through) and put a thick cut of roquefort cheese in the middle.
(Also cited in the December 9, 1951 New Orleans Times-Picayune, sec. 3, pg. 7, col. 4. The original column appeared in the New York Herald Tribune—ed.)
 
Google Books
New Guide to Intelligent Reducing;
How to Reduce and Stay Reduced for Life
By Bengamin Gayelord Hauser
New York, NY: Farrar, Straus and Young
1955
Pg. 149:
It was Irene Castle— the still slim, vital lady— and Pat Dougherty, the society editor who had lost forty pounds by eating intelligently, who introduced me to the Pump Room’s famous butterfly steak. This is a small sirloin split in two with the bone in the middle; it looks exactly like a butterfly. It takes only a few moments to broil such a steak; when it arrived, piping hot, all three of us, in unison, automatically cut off the outside fat.
(The Pump Room of the Ambassador East Hotel, in Chicago, IL—ed.)
     
Google Books
Profitable Meat Merchandising
Progressive Grocer Magazine
1960
Pg. ?:
The last cut or two can be butterfly cut if desired.
 
Google Books
The Gourmet’s Host;
The role of classic cuisine and service in restaurant management
By Paul O. Huebener
New York, NY: Exposition Press
1961
Pg. 49:
BUTTERFLY STEAK
Butterfly steak is cut from either the sirloin or the tenderloin. It is broiled. One of the best known is Butterfly Steak Bordelaise, which is sauteed. Bordelaise sauce is made with Bordeaux wine and sorrel. This steak gets its name from its fancy cut: first, it is cut one inch thick, then, placed flat, it is cut nearly through and opened, and both sides are flattened down so that with a little imagination it looks like the wings of a butterfly.
 
18 June 1961, Sunday World-Herald (Omaha, NE), pg. 7-E, col. 8:
Butterfly Cut Fillets
Add Variety to Fish
New York (UPI)—Ever have your fish fillets cut butterfly style? One of these things consists of two fillets, one from each side of the fish, held together by a “hinge” of uncut flesh.
 
26 April 1962,

(Cleveland, OH), pg. 51 ad:
Butterfly-cut
Boneless Pork Chops lb. 89c
(Stop -n- Shop Super Markets—ed.)
 
Google Books
May 1963, Ebony, “Cordon Bleu Italien,” pg. 144, col. 1:
It is called the butterfly cut taken from the filet of veal.
 
10 October 1965, The Oregonian (Portland, OR), Business, pg. 11, col. 1 ad:
Available in $5, $10 or $15 amounts, they may be redeemed for butterfly-cut tenderloins, ...
(Oregon Beef Council—ed.)
 
Google Books
11 November 1966, Life magazine, pg. 149, col. 2:
If your butcher will do it, have him butterfly the steaks.
 
22 June 1967, Los Angeles (CA) Times, “Elegant Food in the Grand Manner—-the Easy Way” by Cecil Fleming, pg. F1:‎
These are thin meat slices, pounded to tenderness, or butterfly-cut steaks.
 
15 November 1970, The Sun (Baltimore, MD), pg. SM40:
“My butcher calls this a ‘butterfly cut.’”
 
18 April 1974, Chicago (IL) Tribune, “Brazilian national dish” ‎by Ruth Ellen Church, pg. W-A3:
This is a “butterfly” cut. Open out the two steaks on your cut- ting board or a sheet of heavy duty foil and flatten with the broad side of a cleaver.
 
22 April 1993, Orlando (FL) Sentinel “Mediterranean-style stuffed turkey,” pg. H8:
(To butterfly, cut the meat down the center, but not completely through. Open the two halves and lay flat.)
   
Google Books
The Gourmet Cookbook:
More Than 1000 Recipes
Edited by Ruth Reichl, John Willoughby and Zanne Early Stewart
Boston, MA: Houghton Mifflin,
2006,  ©2004
Pg. 437:
BUTTERFLY THE STEAK
 
Le Cordon Bleu
How to Butterfly a Steak
September 8, 2011
One of the most exciting aspects of cooking is learning a new culinary technique that you can use for a variety of different ideas and recipes. The Texas culinary scene is known for its love of BBQ and steaks. If you have ever wondered how chefs get those perfectly prepared thin cuts of steak or chicken you will probably enjoy learning to butterfly meats at home. It will take a little practice at first but you will find that i’s applications are almost limitless.
 
Butterflying is a culinary technique used to turn a thick dense slab of meat into thinner more spread out cuts. There are several reasons why a recipe would call for butterflying. If you are cooking out on a grill and want your meats to cook evenly butterflying your cuts will make them flat and even all over that way you will make level contact with the grill and won’t have to worry about one spot over cooking or staying raw.
 
Recipes or Reservations
July 16, 2012
How To: Trim and Butterfly a Hanger Steak
Hanger Steaks are one of my favorite cuts of beef.  Their texture is similar to Skirt steaks but their flavor is far more complex.