Mine melted as well, just like this, and it’s happened to many others as well. Seems to be some kind of interaction between the EVSE and the outlet. The volt draws a lot of power, so any imperfections in the contact with the outlet generates a ton of heat that ends up melting the wire insulation inside the cord. Most outlets you see these days are the cheapest possible to save money on building costs, so for charging a Volt it’s a good idea to upgrade the outlet you plug into. Also since this happened to you already, your outlet is probably also damaged internally from the excess heat, so I do not recommend plugging anything else into it until it is replaced.

I bought a new GM EVSE on ebay, a newer model that is smaller, and installed this heavy duty hospital grade outlet at the recommendation of others that encountered this problem:

https://www.amazon.com/dp/B019AEBEJW

These are pretty easy to install as long as you turn off the power and have a screwdriver. If you’re not comfortable doing it yourself you can also hire an electrician to install it for you.