If it is not foil, I don't what else it could be (refer replies #1 and #2). It looks just like what happens when foil is applied and the bubble is expanded further. The image of the base shows it looking most metallic. The material looks a bit burnt or tarnished on the upper surface of the vessel, which is one reason to call it 'gold' rather than gold.
It is not cased. The foil material is on the outer surface.
I'm unsure what is a "button rim" (see reply #2) but I did ask myself whether the short neck has been added on or formed by opening out the parent material of the vessel. There is a slightly indented crease around most of the inner circumference of the neck, which might suggest it was added by trailing on. However it is made of the same colour of material as the body of the vessel and there is no longitudinal seam evident, which suggest forming by opening out the bubble. On the outside, the 'foil' material stops sharply at the neck, but that could happen whether the neck was added on or opened out. On balance, I conclude it was simply opened out.