Does anyone know of a hard, heavy, moldable material (like plaster or cement) that is also food safe? I’m looking for something to make bowls and cups without kiln firing like I would have to with clay.
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.
Food-safe implies a lot of things. The container material needs to be non-toxic, it can't break down when containing wet or greasy materials (especially hot), it needs to be washable, it can't have pores or cracks in which microorganisms can grow, it can't contain materials that can leach out and affect the food, it can't be affected by acidic or alkaline foods, etc.
Plaster and cement aren't moldable like clay, but they can be cast into the shape. Plaster is very brittle, so it isn't suitable for this kind of use. Certain cements are used for kitchen countertops and decorative household items, but it isn't fundamentally food-safe. You can seal kitchen countertops to make them more stain resistant, but it is hard to thoroughly seal it short of something like pouring an acrylic layer on top. I wouldn't trust it with a "paint-thin" coating for use in cups and bowls that you eat from.
There are air dry clays made from food products, so they would be non-toxic, but they wouldn't hold up to use as a cup or bowl. You could seal the surface, but I wouldn't trust the coating to last long-term.
There are epoxy clays. I don't have enough familiarity to recommend which specific products are food-safe. You would have short working times and it would be expensive. But that may be as close as you can get to your criteria if you want a clay-like, food-safe material that doesn't require a kiln.
There are a few other materials that could produce cups and bowels that would be food-safe and stand up to handling, but they wouldn't meet all of your requirements.
Oogoo, using food-safe silicone caulk (like used for aquariums), or food-safe two-part casting silicone, is clay-like to work with, but the result isn't rigid like clay. The more clay-like it is, and the harder you make it, the shorter the working time. But it will always be rubbery. That would make it impractical for this use, unless you start with a rigid form and cover it with Oogoo, or use the result inside a rigid liner.
Another option, which is moldable and food-safe, but not clay-like, would be to form it from thermoplastic. Vacuum-forming can get tough with heavy material. Instead, you can soften a heavy sheet or plate and then manually form it around or inside molds. But it's getting far-removed from your concept.