THE BAT SIGNAL. BtVS Intertext/Allusion.
In "Some Assembly Required," Buffy summons Willow to a meeting of the Scooby Gang by announcing "Uh, sorry to interrupt, Willow, but it's the Bat Signal."
The "Bat Signal" is, of course, a floodlight, projecting the bat logo into the sky over Gotham, used by Commissioner Gordon to summon Batman in the DC comic books. Tim Burton's Batman (1989) ends with the Bat Signal's dramatic, iconic first use.