Turns out that you can make diffraction grating* molded onto chocolate if you know what you're doing.
In fact, you can 3D print the lovely effect with all kinds of things.
* in b4 "tHat iS NoT a tRuE HoLoGrAm!!1 U jUsT LoSt a ReaDEr!" Take it up with the video uploader.
Image: YouTube / freckledchemist