From the episode Blood Ties, Season 5 Episode 13:

BEN
Couldn’t find any marshmallows. I’ll try 
to steal some for next time.

DAWN
(absently)
Don’t like ‘em anyway.

BEN
(laughs)
What? Is that even possible?

DAWN
Too squishy. When I was five, Buffy 
told me they were monkey brains…

From the episode Conversations With Dead People, Season 7 Episode 7

ANGLE ON DAWN: On her tummy in front of the television, which is emitting creepy horror-movie music, lit by the TV light and licking marshmallow goo off her fingers as she talks on the cordless phone.