A demon wins from Shiva the power to destroy whoever he touches on the head and turns on the giver; Vishnu, as Mohini or as a young brahmachari, tricks him into testing the boon on himself.
Bhagavata Purana 10.88 — Vrikasura and the brahmachari
Pan-IndiaVaishnava
The Bhagavata sets the story inside a question: why are Shiva's devotees rich and Vishnu's poor? The demon Vrika asks Narada which god is quickest to please and is told Shiva, with Ravana and Bana as proof. At Kedara he offers pieces of his own flesh and on the seventh day is about to cut off his head when Shiva stops him and grants his wish: death to anyone he touches on the head. Vrika reaches for Shiva's head at once; Shiva runs, and none of the gods can help until he reaches Vaikuntha. Vishnu comes out as a young student, greets the demon sweetly, tells him Shiva has grown unreliable since Daksha's curse, and suggests he try the boon on his own head to check it. The demon does, and his head bursts.
What is different here:
Base telling. No Mohini and no dance; Vishnu's disguise is a brahmachari, and the demon is Vrika, not Bhasmasura.
Regional Bhasmasura telling (dance and performance)
South IndiaFolk
In the regional telling the demon is Bhasmasura, born of Shiva's ash, and his boon reduces to ash whoever he touches on the head. When he turns on Shiva, Vishnu comes as the dancer Mohini and offers to dance with him if he can match every pose. Step by step he mirrors her until she rests her hand on her own head; he copies the gesture and burns to ash. The scene survives on the Yakshagana stage, in Bhojpuri women's dances and in Indo-Caribbean chutney dance, and in Kathakali it is one of the central narratives. Pattanaik adds that some regard the story as a Vaishnava composition contrasting a shrewd Vishnu with a gullible Shiva; attribute that reading to him.
What is different here:
Demon renamed Bhasmasura; Vishnu's disguise is Mohini and the trick is a dance; Wikipedia says the character is absent from the Puranas.