What is the episode and season in which Buffy is wearing "Yummy Sushi Pyjama's??"

I remember seeing this episode on TV where Buffy was trying to sound really smart and then she says something like, "that would have sounded better if i werent wearing my Yummy sushi pyjama's. That episode made me laugh and has always stuck in my head but I cannot remember which season or episode it is. Can somebody help me???