Correspondence chess built on top of the scuttlebutt platform. More information about scuttlebutt here: https://staltz.com/an-off-grid-social-network.html and https://www.scuttlebutt.nz/
It is built to allow it to be integrated into scuttlebutt viewers (such as patchbay, patchwork using depject so that they can take care of things like discovering friends to play with, etc.
You can find a desktop app releases to run ssb-chess at https://www.github.com/happy0/ssb-chess-electron.
SEe https://github.com/Happy0/ssb-chess/blob/master/README.md#required-ssb-server-plugins for information about ssb-server plugins required to run this.