Here is an article on the topic of Gopichandana and Bhasma dharana with Pramanas from the Vasudeva Upanishad and the Brihajjabala Upanishads. Tamil texts too are cited. https://adbhutam.wordpress.com/wp-content/uploads/2024/11/fie-upon-the-forehead-that-is-devoid-of-bhasma-f-1-1.pdf