That’s a good question. From what I can tell, the animating idea behind it is similar to that of a flourless chocolate cake: a dense confection for those times when conventional chocolate cake simply isn’t enough. Though this cake is lighter and was probably invented earlier. Nearly all of the recipes I’ve found call for melted bar chocolate in the mix, and usually cocoa powder besides. Many have a topping. The American versions — which I’m noticing are often called “Mississippi mud” cakes — are generally topped with marshmallow, the European versions with meringue.
Another thing that unites most mud cakes is the mixing. Almost all of them employ the muffin method, the wet and dry ingredients are mixed in separate bowls and then gently combined before panning and baking. Very simple, in other words. I’m starting to see why these are so popular in places!
Here I should insert that “Mississippi mud” cakes are very different from “Mississippi mud” pies, the latter being very sweet custard-type chocolate pies baked in chocolate cookie crumb crusts. Neither one from what I can tell hails from the state of Mississippi or any other state along the river’s banks. My guess is that the “mud” cake idea came first, and “Mississippi” was added later in an effort to make the name a little more fun and musical. Ain’t it neat to beat your feet on the Mississippi mud…
I’d go further and say I can’t be sure at this point whether this cake is even American originally. The chemical leavening and overall richness and thickness would suggest that, but really…who knows? If anyone can find any evidence either way I’d be interested. So far I’ve found next to nothing!