A “stud muffin” (or “studmuffin”) is a very attractive man (a “stud”). The “muffin” ending is less understandable, and it possibly is related to the similar term “beefcake.”
 
The origin of the term “stud muffin” is unknown. “The Stud Muffin Revue” was printed in the Nashua (NH) Telegraph on December 16, 1976. “Studmuffins” was printed in a Valentine’s Day personal ad in the Indianapolis (IN), Star on February 14, 1978.
 
     
Wiktionary: stud muffin
Noun
stud muffin (plural stud muffins)
1. (idiomatic, colloquial) A very attractive man.
 
Wiktionary: studmuffin
Etymology
stud +‎ muffin; perhaps with influence from beefcake.
 
(Oxford English Dictionary)
studmuffin, n.
colloquial (originally and chiefly North American).
A sexually attractive man, typically one who has a muscular physique.
[1979   G. F. Schumm Teaching Compan. to Lemmon’s Beginning Logic iv. 39   All the cats, save Joychunks and Studmuffin, used the litter box.]
1986   D. Barry in Chron.-Telegram (Elyria, Ohio) 9 Jan. d11/3   In Washington, House and Senate budget conferees agree to call each other by nicknames such as ‘Stud Muffin’.
1986   N.Y. Times 26 Nov. c22/2   Howard Busgang comes on [stage] as a Woody Allen-type nerd, advising other ‘stud-muffins’ that ‘you’ve gotta go where the women are vulnerable.’
   
Newspapers.com
16 December 1976, Nashua (NH) Telegraph, pg. 35, col. 4:
THE
WILLOWDALE SPA
TYNGSBORO
Presents
DIRECT FROM
LAS VEGAS
(...)
ALSO
FOR YOUR DANCING PLEASURE
THE STUD MUFFIN REVUE
 
Newspapers.com
14 February 1978, Indianapolis (IN), Star, pg. C-1, col. 5 classified ad:
STUDMUFFINS-
Twinkling eyes, Gentle touches. You’re the sweet guy I love so muches. Forever, Buddy
 
Newspapers.com
14 February 1979, San Francisco (CA) Examiner, “Happy Valentine’s Day,” pg. 37, col. 9 classified ad:
STUD muffin, bring your big beautiful body around more often. I love you, Lovee
 
Newspapers.com
14 February 1981, Democrat and Chronicle (Rochester, NY), “Happy Valentine’s Day,” pg. 7B, col. 4 classified ad:
STUD MUFFINS
Roses are Red
Violets are Blue
My Heart SOARS
When I’m with You!
I Love You, Linda
 
Newspapers.com
23 February 1984, L.A. Weekly (Los Angeles, CA), “L.A. Dee Da,” pg. 35, col. 3:
... when they were joined onstage by the fabulous Joe “Call Me Studmuffins” Berardi for a cookin’ version of Marvin Gaye’s “Innercity Blues.”
   
23 May 1993, New York (NY) Times, “On Language: Stud-Muffin’s Buzz-Kill” by William Safire, magazine sec., pg. 12, col. 2:
A MUFFIN IS AN affectionate name for a young woman—“Not now, muffin”—usually taken as a put-down by the daughters of what a previous generation called cookies. In a vengeance expressed in campus slang, we now have stud muffin, which means “attractive young man.” (The stud is rooted in the term for a male horse selected for breeding.) A stud muffin who works out, I am informed by Charles D. Poe of Houston, can transform himself into a diesel.
       
OCLC WorldCat record
Studmuffins of science : 1996 calendar
Author: Karen Hopkin
Publisher: [New York] : [Studmuffins of Science], 1995.
Edition/Format:   Print book : Biography : English
 
OCLC WorldCat record
Studmuffin
Author: Marilyn Duckworth
Publisher: Glenfield Vintage 1997
Edition/Format:   Print book : English : 1st publ
Summary:
Alice, 32-year-old ex-teacher, slips through the “Looking Glass” hand in hand with an albino accountant. Together they must pit their wits against the sinister Powell, who is a cross between a modern Mad Hatter and a malignant Wizard of Oz.
   
OCLC WorldCat record
Render me, gender me : lesbians talk sex, class, color, nation, studmuffins.
Author: Kath Weston
Publisher: [Place of publication not identified] : Columbia University Press, 1998.
Edition/Format:   Print book : English
Summary:
In day-to-day life, people often act as if they know exactly what they mean by boys and girls, masculine and feminine, butch and femme. Render Me, Gender Me challenges comfortable assumptions about gender by weaving Kath Weston’s own thought-provoking commentary together with the voices of lesbians from a variety of race and class backgrounds
 
Urban Dictionary
studmuffin
A rather attractive human of the male variety. Can be described as being ‘good enough to eat’.
mmmm…. I can smell freshly baked studmuffin.
I had the best studmuffin last night.
#stud#muffins#stud muffin#cute guy#sexy boy
by Becmond November 14, 2005
     
OCLC WorldCat record
Stud muffins : luscious, delectable, yummy (and good muffin recipes, too!)
Author: Judi Guizado
Publisher: Guilford, CT : Globe Pequot Press, ©2008.
Edition/Format:   Print book : English
Summary:
A full-colour cookbook featuring creative muffin recipes with unforgettable serving suggestions, ‘Stud Muffins’ radiates the kind of heat you actually want in the kitchen.
 
OCLC WorldCat record
Studmuffin Santa
Author: Tawna Fenske
Publisher: Toronto : Tawna Fenske, 2017.
Edition/Format:   eBook : Document : Fiction : English
Summary:
Brandon Brown is the unlikeliest Santa ever to don a red velvet suit. His abs are more cheese grater than bowl full of jelly, and deploying to Syria is the closest he’s come to the North Pole. But family drama sends the jaded Marine home for the holidays, and lands him an unexpected gig as a part-time Santa. Jade King is hustling to get Jingle Bell Reindeer Ranch off the ground, and she’s not thrilled her sister hired a Santa with sex-appeal. The last thing Jade wants is a beefcake circus, and guys like Brandon bring sleigh-loads of trouble. But Studmuffin Santa proves more popular than anyone dreamed, drawing hordes of happy kids, lusty moms, and a mystery foe hell-bent on wrecking it all.