I think the Ensō[*] version of this would be a single piece of paper and a pen filled with ink which disappears after 10-15 seconds.

Does anyone know if such ink exists or if there's a way of making one cheaply/safely? I want to build prototype and record a video.

(* Ensō is a stream of consciousness writing app I built some time ago, hence my very clever idea: https://enso.sonnet.io)