I've checked some sources on this. It mimics MOCVD, Molecular Organic Compound Vapor Deposition. The only thing is MOCVD is usually done at a .01 mm of Hg. I guess if the peanut butter acts as an directional membrane you could get the pressure needed when the charcoal cools.

I do know for fact you can make synthetic diamond thin films by evaporating graphite then condensating it. That's what they do with the MOCVD reactor.