Young female protagonist (high school-ish) has the unique ability to grow a penis (that only she can see) on any surface. She uses this ability to get herself off every chance she gets. The manga culminates in her using the ability on a passenger on a subway thinking that HIS dick is one that she grew, eventually ending in a double penetration (his and one of her visible dicks).
I haven't been able to find this after more than a year of searching various tags on various websites. Kinda frustrating, honestly.