This was a frequent topic of discussion in the past, but it's a different discussion when a US unlicensed traveler actually has a stamp in the passport that specifically identifies Cuba. Without a stamp there was no reason to list Cuba on the customs declaration form or volunteer that the traveler had been to Cuba. It is certainly more problematic if there is a stamp.
The statute that Matt mentions is 18 U.S.C. 1001 which says:
whoever, in any matter within the jurisdiction of the executive, legislative, or judicial branch of the Government of the United States, knowingly and willfully—. . .
(2) makes any materially false, fictitious, or fraudulent statement or representation; . . .
shall be fined under this title, imprisoned not more than 5 years
About 7 years ago we had a three page thread debating whether failing to list Cuba on a customs declaration form was a violation of this statute. I personally think it is, but I also know that the chance of a person getting prosecuted under this statute for failing to list Cuba really is "close to zero." But even if the chance of prosecution is slight, it doesn't give much guidance about what a traveler with a Cuba stamp in his/her passport should do.
I think the best course is the one I suggested. List Cuba and if questioned say you thought that it was no longer illegal to travel to Cuba. You are probably going to be let off with a warning, but at worst you are looking at a minimal fine if the CBP officer decides to report the trip to the OFAC and they then decide to pursue a civil violation.
I agree that the problem is compounded if the officer believes you were trying to conceal the trip. The officer's opinion about how you responded to his questions would be included if he decides to report the incident to the OFAC. It could also factor into how the OFAC chooses to proceed.
Again I think the likelihood of anything more than a few disquieting minutes at immigration is slim, but this is not something that should be treated in a cavalier manner.
Good luck and do report back.