This lovely wunderkammer piece was done by the fabulous Propnomicon a couple of years ago, but it just bubbled back up on Pinterest.
It’s a “vampire heart” complete with stake hole, along with the offending stake; they’re packed in a wooden box and include a backstory involving a vampire that stalked Louisiana in the 1900s. His posts about it are here: Part 1, Part 2, Part 3.
He’s got photos of the finished prop and a few vague notes about the build (noting that it involved liquid latex and followed some techniques used to build sideshow gaffs), but unfortunately doesn’t have any pictures of the work in progress nor details about the construction. (A couple of his other posts indicate that he might build these on commission if you can’t live without one.)
Part of me is almost tempted to see whether it would be possible to achieve something similar by mummifying a pig heart (sometimes found at ethnic groceries; beef hearts are easier to find but they’re too big). I’ve covered DIY mummification previously, and if the heart were carefully trimmed of fatty tissue and well-packed with a drying agent it should get hard and leathery. The only thing I’m unsure of would be its long-term durability; treating it with a taxidermy rot preventive like Stop-Rot might help with that. Since the materials are fairly cheap, it might be worth an experiment.