Modern molds all seem to be plastic. Does anyone
make new molds out of metal?

Antique molds are metal, they usually appear to be
stamped tin. Some appear to be steel (because they
have rust). I believe Reiche silver-plated their molds.

What would an ideal mold material be? I think the
base material should be copper, aluminum, or silver,
for heat conductivity, ductility, and cost. I think the
surface material probably should be gold, for lack
of reactivity with chocolate, lack of corrosion from
washing and storing wet, and lack of adhesion (gold
is a very non-stick surface). This implies that there
must be a barrier layer between the base metal and
surface layer, so the gold won't diffuse into the base
metal. A thin (less than half a mil) nickel layer would do.
For a deluxe mold, it should be electropolished
for a mirror-bright finish.

Plastic molds are too thick for good heat conduction
during cooling, they emit unreacted monomers and
other chemicals, they absorb fat soluble molecules
(like many flavors and aromas) then emit them into
the next batch, and they can't be given a bright mirror
surface like metal.

Are there any serious mold makers who already
do things the right way? Or would take an order
for a custom metal mold made to a demanding
specification?