I'd get a cookie cutter in the shape of, oh, I dunno, a gingerbread man. Stick it on her ass, cum into that, smear it out evenly and wait for it to dry. Remove cookie cutter, and give her a gingerbread man shaped slice of cum fresh off her ass.
It might take some time. How much does she charge for the hour? I suppose a hairdryer would speed things along. Would a hairdryer help the cum go hard faster? I'm sure someone on here must know.