Fuck knows. Anything flammable that can be layered on top of kahlua. It's not like it's supposed to taste nice. Chartreuse does have that sinus-clearing effect, of course.
Bacardi 181 generally works well for this too ... it's what enables the "burn" in a "Burning Desire". :-)
Always found Drambuie (sp?) burns like a good 'un - Drambuie Flamers [1] having been a great downfall of mine in the past
Plus it doesn't taste that bad [2] with the Kahlua ...
[1] Swill shot(s) in glass, light, cover with hand till glass sticks to hand, remove hand just enough to suck fumes out, down in one, and pour remaining drops onto hand to snort. (Don't think last bit does anything, but by god you get strange looks ...)
[2] For varying values of $bad