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Diffraction gratings are composed of a series of ridges spaced very closely and regularly, and this physical shape is transferred to the chocolate through the molding process. Light bouncing off of the different ridges creates an interference pattern which varies for different wavelengths of light, creating a rainbow effect.

https://en.m.wikipedia.org/wiki/Diffraction_grating

Google for a "reflective diffraction grating". If you shape the surface a certain way - grooves with a size near the visible wavelength, AFAI understand - you can get the reflected light to interfere with itself. The imperfections which vary the groove size then result in this rainbow effect.

It’s moulded. The vacuum helps to make the end result hold the shape better.