How Ganesha got Elephant Head according to Brahma Vaivarta Purana?

A less well-known story from the Brahma Vaivarta Purana recites a different version of Ganesha's birth caused due to Gaze of Shani (Shani Vakra Dhrusti). On the insistence of Shiva, Parvati fasted for a year (punyaka vrata) to propitiate Vishnu so that he would grant her a son. Lord Krishna, after the completion of the sacrifice, announced that he would incarnate himself as her son in every kalpa (eon). Accordingly, Krishna was born to Parvati as a charming infant. This event was celebrated with great enthusiasm and all the gods were invited to take a look at the baby. However Shani (Saturn), the son of Surya, hesitated to look at the baby since Shani's gaze is said to be harmful. However Parvati insisted that he look at the baby, which Shani did, and immediately the infant's head fell off and flew to Goloka. Seeing Shiva and Parvati grief stricken, Vishnu mounted on Garuda, his divine eagle, and rushed to the banks of the Pushpa - Bhadra river, from where he brought back the head of a young elephant. The head of the elephant was joined with the headless body of Parvati's son, thus reviving him. The infant was named Ganesha and all the Gods blessed Ganesha and wished Him power and prosperity. Another tale of Ganesha's birth relates to an incident in which Shiva slew Aditya, the son of a sage. Shiva restored life to the dead boy, but this could not pacify the outraged sage Kashyapa, who was one of the seven great Rishis. Kashyap cursed Shiva and declared that Shiva's son would lose his head. When this happened, the head of Indra's elephant was used to replace it. Still another tale states that on one occasion, the used bath-water of Parvati was thrown into the Ganges and this water was drunk by the elephant - headed Goddess Malini, who gave birth to a baby with four arms and five elephant heads. The river goddess Ganga claimed him as her son, but Shiva declared him to be Parvati's son, reduced his five heads to one and enthroned him as the Controller of Obstacles (Vigneshwara).