Warm butter and sugar in a saute pan over medium heat until butter melts, 1 to 2 minutes. Add bananas, cinnamon, salt, nutmeg, and vanilla, and cook, stirring, for 1 to 2 minutes. Remove pan from heat. Add rum. Return to heat. If using a gas stove, tip pan away from you carefully until the vapors from the rum ignite. (Or light with a long match.) Cook until flames stop, about 1 minute. Divide bananas among crepes; reserve sauce in pan.
Fold each crepe in half, then fold in half again. Drizzle crepes with reserved sauce, and top with creme fraiche or ice cream. Serve immediately.