Daniel H. answered 01/15/23
Over fifteen years in Film & Video, Specializing in Post-Production
Yes. The actor who is singing must actually be singing the song twice as fast as it would otherwise be (the on-set music person would double the time signature).
Then, the DP would shoot the scene at an increased speed (double the normal 29.97 fps or 23.98 fps, so usually close to 59.94 or 60 fps, but sometimes closer to 48).
Playing it back like this will put Buffy in slow motion, while the song should appear at normal pace. This was also done on “Glee” in the episode “Wheels” from Season 1.