The compound "butter-flavoured" is an adjective, as illustrated thus:
Should that be:
1. abnormally butter-flavored nipples,
2. abnormal buttery-flavored nipples,
3. abnormal butter-flavored nipples, or, god forbid,
4. abnormally buttery-flavored nipples.
"Wirthling's nipples are pink."
"Wirthling's nipples are cracked."
"Wirthling's nipples are butter-flavoured."
"Abnormally" modifies an adjective that follows it (well, technically it is an adverb, and it applies to the "is" or "was" or "are" or whatever verb precedes the phrase) and since we have seen that "butter-flavoured" is an adjective, the correct form of the phrase is...
The Non Sequitur Donkey hands DexX the envelope.
...and the winner is, "Wirthling has a face that only a llama would refer to by its maiden name."
Hope that settles it.
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