Actually Jackie, you are nearly right about the bain-marie concept.

I once saw a boiled caked on a show called La Cuisine des Mousquetaires (The Muskateer's Cuisine). The two hosts prepared food exactly how it had been during the time of the muskateers.

The cake batter is poured into a deep pot, which is put inside a wider pot of nearly the same height. Water is poured into the the bigger pot and the water is boiled until the cake is done.

The cake batter has to include much less liquids than a regular cake batter since there is a lot of humidity created by the boiling.

It was the only way that people could get a cake when they cooked over fire pits.