Food Safe Plaster-Like Material
Food safe, moldable materials are rare in general. The closest thing that comes to my mind is a kind or "organic plastic" made from milk, but it looked hard to work with and I don't know how food safe that really is on the long run.

Take for example a simple porcelain dish: the porcelain itself is porous and would stain with food residue. It's the glaze that makes it food safe.

I suggest using whatever clay is best suited for the style of bowls you want to create, then applying a food safe varnish or lacquer in several coats to seal the surface.