It's a deleted scene from "Surprise".
I should have been more specific. I would have loved to have seen it. And there was another one of the same sort but between Xander and Willow:
Xander and Willow are in the school lounge at the snack machine.
Willow: So we’re all set. I’ve got all the decorations. And I think Cordelia’s bringing snacks and stuff-
Xander: Yeah. She said she was. Which was thoughtful. Don’t you think? Hey. Cordelia: not as horrible a person as we once though? I mean, she’s obviously trying to be helpful.
Willow: True. (considers) Maybe…
Xander: But, you wouldn’t ever, like, be able to be friends with her or something.
Willow: You mean, like hang out and take Cosmo tests together or something?
Xander: I mean – actually elect to be in her presence.
Willow: I don’t know. She’s better – but she’s still Cordelia. Just… look.
Cordelia and Harmony are joined by a few other Cordettes. They all squeal in greeting, jumping up and down.
Willow: Example: what is the shrieking thing? They just saw each other yesterday… And now, watch – Cordelia’s going to model her new outfit-
Cordelia is spinning around, showing off her mini-dress as the Cordettes shriek some more.
Willow: Note the reaction – like Cordelia invented clothes. They’re not bad people, Xander. It’s just – we are of two worlds. And theirs is bad.
Xander: (letting it sink in) Right. Of course you’re right. What was I thinking?