In later Latin, as ipse started to lose its force, Petronius uses ipsimus for emphasis:
Tamen ad delicias ipsimi [domini] annos quattuordecim fui. Nec turpe est, quod dominus iubet. Ego tamen et ipsimae [dominae] satis faciebam. Scitis, quid dicam: taceo, quia non sum de gloriosis.
Well, I was my very own [master]'s beloved at age fourteen. It's not shameful, whatever your master orders. And, well, I also used to do enough for my very own [mistress], too. You all know what I mean—I won't say any more, because I'm not the boastful type.
(Translation mine—this is from the mouth of Trimalchio, who uses intentionally bad Latin at times.)
Much more recently, Aleister Crowley uses a form ipsissimus in his esoteric writings:
And this is the Opening of the Grade of Ipsissimus, and by the Buddhists it is called the trance Nerodha-Samapatti.
Both seem like standard superlative forms from ipse, treating it like a normal adjective.
But are either of these good Classical style? Is there a "correct" superlative form attested in non-Vulgar Latin? And if so, which is it?