Lover's Walk - S3-E8
Corrected entry: Buffy is talking to her mother about college while making a bag of microwave popcorn. When the microwave goes off, she takes the fully-popped bag out and opens it. When it opens, there is no puff of smoke from the heated oil in the bag, indicating that it was pre-popped for the scene.
Correction: I've made microwave popcorn many times and have never seen a puff of smoke. Perhaps the puff of smoke occurs when overheating his/her popcorn.
Carl Fink