Sarah Michelle Gellar is known internationally as "Buffy the Vampire Slayer" and starred in the series from 1997 to 2003. Buffy is one of the many shows that the male cast of "The Big Bang Theory" was obsessed with.
She was mentioned as Buffy in "The Romance Resonance" in the song "If I Didn't Have You" written by Howard for his wife. In "The Parking Spot Escalation", Raj mentioned the comic book that continued the series. Leonard also tried to get Penny interested in the series so that they would have something else to share.
In "The Stockholm Syndrome", Raj flies to Stockholm seated next to Sarah Michelle Gellar. Howard doesn't believe that it's her since she is asleep. Next she is seen seated next to Raj at the Nobel Prize ceremony. She must have agreed to attend, though when she finds Raj holding her hand, she reminds him that they aren't on a date and shows him her wedding ring. During his speech, Sheldon spots her from the stage and he asks if that is Buffy. Sarah waves to him and Sheldon waves back.