This doesn't actually need a recipe - at least I've never used one.
Cook your yams, you can either bake them or boil them.
Remove skins and mash the yams along with butter and whatever quantity of brown sugar suits your palate.
Scoop into a baking dish. (Deep will give more yam per scoop, shallow will give more marshmallow per scoop.)
Top with marshmallows. (I prefer mini marshmallows kind of piled on)
Stick under the broiler for a minute or two until the marshmallows melt, puff up and turn golden.
You can play with this a lot. For instance, some orange juice added to the mashed yams gives a different flavour. You might prefer jumbo marshmallows and lots of them, or mini and not many. A bit of nutmeg in the yams maybe. The yams do not need to be timed to be cooked just as everything else is, but kept warm until ready to be mashed. Puffing the marshmallows doesn't take long, no longer than meringue for sure.
If there's a recipe out there, I will be interested in seeing it. This is was a standard dish on our thanksgiving table when I was a child and this is how I learned to cook it.