Pops up occasionally in Discworld. There was something similar to "Its eyes were as big as very big eyes"; lampshaded, in that the creature's eyes were traditionally described as being "as big as soup plates", but Tiffany had measured a soup plate and determined that they weren't. A substitute description was needed.
Pratchett really likes these "what's-that-thing" quips. Sourcery has several.
"What's dat fing? Dey goes all crumbly when you eat dem?"
"... could be a lawyer."
"Dey goes soggy if you dips them in somefing?"
"More likely to be a biscuit, then?"
(Some trolls have the full "intelligent but cannot properly express ideas" Buffy Speak trope, though others... don't.)
"Limited wossname. Doodah. Thingy. You know. It's got words in it."
"The thing that went 'parp' went parp."
Dwarfs, who are too literal to understand simile or metaphor, do this all the time.
Carrot (speaking about his dwarf girlfriend): "She's got a beard as soft as a very soft thing."
"She was as thin as a very thin thing."
There was also law passed by a former Patrician about metaphor and the like. If you're going to say a girl has "a face that launched a thousand ships" she'd damn well better have a champagne bottle for a head.
Granny Weatherwax offers this fine example, which is actually an excellent observation, in Wyrd Sisters: "Things that try to look like things often do look more like things than things." (She's talking about stage-prop crowns versus a real crown; It Makes Sense in Context.)
In Men at Arms, Gaspode demonstrates how a large vocabulary doesn't always mean you can come up with the right word (he's trying to say that dogs don't care about clothes):
[']Clothing has never been what you might call a thingy of dog wossname.' Gaspode scratched his ear. 'Two metasyntactic variables there. Sorry.'
The Confessions of Georgia Nicolson series uses this trope constantly: "as fine as two fine things on a fine day out in Fineland", "staring like a staring thing", "as mad as two very mad things", "loony as a loon on loon pills."
The Georgia Nicolson books often refer to "snognosity". Not classic Buffy Speak, but definitely related.
"He abused my hospitality," booomed the earl. "I swore that if he ever again entered my domain I would have him gutted and dried like, like something that had been gutted."
Bertie Wooster (in both books and film/TV adaptations) frequently finds himself in the middle of an aphorism he can't complete without Jeeves' help. (The books are almost always narrated by Bertie but with a brilliant, effortless prose that a goof like Bertie would never be able to manage in real life and yet nonetheless seems plausible while you're reading it.)
Bertie: Let me tell you that a man without music in him is fit for... excuse me a moment. Jeeves, what was it Shakespeare said a man without music in him was fit for?
Jeeves: Treasons, stratagems, and spoils, sir.
Bertie: Jeeves, have you seen that play called I-forget-its-dashed-name?
The hero of H. P. Lovecraft's "Beyond the Wall of Sleep" is a man whose doctor recognizes as being possessed by some cosmic entity of superhuman intelligence who is struggling to express its metahuman thoughts and ideas through the man's stupid and backwoods brain and vocabulary.
"Big, big cabin with brightness in the roof and walls and floor, and the loud queer music far away..."
In That Hideous Strength, women are described as being able to "speak a language without nouns" when there are no men around and still be understood:
"If two men are doing a bit of work, one will say to the other, 'Put this bowl inside the bigger bowl which you'll find on the top shelf of the green cupboard.' The female for this is, 'Put that in the other one in there.' And then if you ask them, 'in where?' they say, 'in there, of course.'"
In The Warlock by Michael Scott, Virginia Dare saves Billy the Kid from a 'raggedy lion-monster-thingy'. It's a sphinx.
Roswell High certainly uses this. For instance, from the first chapter of the first book "Guys. I'm so tired of their... guyness." Also, this series sort of becomes an interesting example of Buffy Speak, as it even uses the word "wiggins" a few times.
Don Quixote: Sancho doesnt really understand that the insula he was promised as a Standard Hero Reward means an isle, as we see at Chapter II of the Second part:
"May evil insulas [islands] choke thee, thou detestable Sancho," said the niece; "What are insulas [islands]? Is it something to eat, glutton and gormandiser that thou art?"
"It is not something to eat," replied Sancho, "but something to govern and rule, and better than four cities or four judgeships at court."
The first official description for the Warrior Cats book The Ultimate Guide describes the book as having an "oversized, gift-y trim", whatever that means.
A Ramona Quimby book acknowledges this trope from Ramona's point of view, when she's told not to keep using the word "stuff" and can't see what's wrong with it:
Stuff was a perfectly good, handy, multipurpose word and easy to spell, too.
Harry Dresden does not do this. He explains himself fully, and if he brings up a word that people don't understand, he takes the time to explain it in simple language. His apprentice, on the other hand...
Not completely true. In Fool Moon he uses the phrase "Don't mess with a wizard when he's wizarding!" Though given the circumstances, it's forgivable.
Even Charles Dickens gets in on this a bit: In A Tale of Two Cities, the narrator informs us that Charles Darnay has been accused of revealing information to the French "wickedly, falsely, traitorously, and otherwise evil-adverbiously."
"Today was a very cold and bitter day, as cold and bitter as a cup of hot chocolate, if the cup of hot chocolate had vinegar added to it and were placed in a refrigerator for several hours...The stranger was a woman, at least as tall as a small chair and probably as old as someone who attended nursery school many years ago."
Rose Hathaway from Vampire Academy, regularly employs this. She names "Rose-logic" her method of finding "loopholes in everything in order to rationalize doing something unreasonable".
This Lyttle Lytton Contest winner: "The general, one might have said, had a sly, sneering-smile expression upon his face."
Randall Monroe, creator of xkcd, has written a non-fiction book "Thing Explainer" which explains complex things (like helicopters) using only the 1000 most common words in English. This makes it a non-fiction book written entirely in Buffy Speak. (See xkcd entry in Webcomics section for the original inspiration for this book.)