Is custard good for dogs?
Probably not, because custard has sugar, and sugar is not good for dogs.
All substances can evaporate. If custard were exposed to a vacuum, all the water would evaporate off, but the more solid parts may remain. Custard doesn't evaporate when heated because it burns at a lower temperature than it it evaporates, and a good chef never cooks custard high enough to burn anyway.