I don't think it is mentioned as the supreme form of Lord Ganesha but it's worship is different from the usual worship. Because it is done (as the word Ucchistha already indicates) in an impure state viz - after not being washed after eating.
This is mentioned as a secret Tantric method.
sArabutamimam mantram na deyam yasya ksyachit |
sarvAgameshva hitabudhyA prakAshitam ||
Na tithirna cha nakshtram
nopavAso vidhiyate |
Yatheshtha chintayA mantrah
This essence-of-all mantra is kept secret in all the Tantras but revealed thinking
about the world's benefit. In this mode of God's worship, there are no
rules pertaining to Tithi-Nakshtra etc, neither fasting etc are
required. Whichever desires the aspirant has while worshipping
Ucchista-Ganapaty are all fulfilled.
The Rishi etc for the mantra you have mentioned is as follows:
Shirasi sudhira rishayae namah mukhe nichrid gAyatri chandase namah
hridi ucchishtaganapataye namah ||
Rishi-Sudhira; Chandah-Nichrid GAyatri; DevatA-Ucchishta Ganapaty.
DhyAnam for this form is as follows:
Raktamurti ganeshancha sarva-Abharanbhushitam |
trinetrancha raktapadmAsane sthitam ||
dvidantam sasmitAnanam |
Ishtancha dakshine haste dantancha
PAshAnkushau cha hastAbyAm jatAmandalaveshtitam
LalAtachandrarekhAdhyam sarva-alankArabhushitam ||
The murti of the deity is of red-color; is decked with all kinds of
ornaments; is wearing red-color garments. He has three eyes and is
seated on a red lotus. The deity has four arms, his body is huge, has
two tusks and the face is ever-similing. In the four hands, he is
holding - vara-mudrA, a tusk, pAsha and an ankusha. The deity has jatA
in his head and is having a crescent in the forehead as well.
After meditating upon the deity as having such a form, it is to be worshipped. 16,000 chants are required for Purascharana of the mantra.
The Sadhana is to be done spanned over Krishna Chaturthi to Shukla Chaturthi. And, as per some Tantras, a Shakti (a female co-sadhaka) is required in it.
And, the mantra Japa is always to be done while not being washed after eating.
Bhuktocchistha japennityam ganeshoham
According to Garga Muni one should do the Japa in a secluded forest being smeared with red sandal paste and while chewing betel leaves.
Anyamate sampujya modakam charvayan bhrigumate phalamashnan |
Vibhishanamate mAmsanaivedyam datva tadeva khAdayan ||
According to other Tantras, while chewing modakas, one should mentally
chant the mantra; according to Bhrigu Muni while chewing fruits.
According to Vibhishana, after offering naivedya meat to the deity,
Japa to be done while eating that as Prasada.
Besides, there are many many other benefits of this Sadhana.
All the quotes are from Brihat Tantra SAra's (2010) revised Edition.
After seeing the following comment, I have gone through various Ganesha mantras found in TantrasArah (TS) and other books like Sarva Deva-Devi mantra kosha (SDDMK).
sriman, i heard about another avastha(sankalpa) having Goddess
sitting in Ganesha's lap...with Anar fruit in one of his hands! this
prompted me, am curious :-) , to get more about this form!
The codes for Ucchista-Ganapaty mantra is given in both TS and SDDMK and is as follows:
Om hasti-pishAchini khetadwayam |
TantrAntare: Hastpadam samucchArya pishAchini padam tatah | DevarAjam
sanetrancha kAntamishaswarAmbitam | Vahni-jAyAvadhirmantra-starAdyah
sarvakAmadah | Pranava sthAne gamiti kechit | Hasti-pishAchini kheha
agnivanitA gam tadAdita ..
The author recovers the codes that are he found in Tantras for the mantra.
Om hasti-pishAchini khe swAhA is the mantra but as per some Gam hasti-pishAchini khe swAhA is the mantra.
Lord Ganesha has many Mantras of which I have listed few below. I have checked the DhyAnams for all these forms.
Mahagnesh mantra 1 - Om srim hrim klim glaum gam ganapataye ... vashamanaye swAhA ..
... Trinetram rasAdAshlishtam priyayA sapadmakarayA sAngasthayA santatam ...|
So, here Ganesha is being embraced by his consort, who is seated on his left. But this form is not Ucchista-Ganapaty.
Mahaganesha 2 -
Hrim gam hrim mahAganapataye swAhA (12-letter)
In this form too on Lord Ganesha's lap his Shakti (consort) is seated, who is padma-kAntA vishishta (complexion is like that of lotus).
MahAganapaty 3 -
Om hrim gam hrim vashamAnaya swAhA (11-letter)
Here too, Lord Ganesha is touching his consort's person with a lotus, and here the consort is dark-complexioned and she is holding a lotus.
Heramba mantra -
Here, Lord Ganesha is not accompanied with his Shakti.
Haridra Ganesha mantra -
Here too, Lord Ganesha is not accompanied with his Shakti.
In the Ucchista-Ganapaty form too, he is NOT accompanied by his Shakti as is shown in the DhyAnam.