I use a square 9" X 9" pyrex casserole dish, but I don't think it matters what you use all that much.
Ingredients:
A bunch of sweet potatoes. I usually need five or six good sized sweet potatoes for the 9" X 9" dish - adjust upwards or downwards accordingly. Look for nice, straight, not too big in diameter sweet potatoes - working with those is a lot easier than working with the fat kinky ones. Yams will work too, but I personally like sweet potatoes better.
Butter - I use about half a stick's worth for this size casserole dish. Sweet butter is good because it doesn't have any sodium and keeps the sodium content of this dish to an absolute minimum.
Brown Sugar - I usually end up using about half a cup, I guess. I never really measured it out, though, so that's a pure guesstimate.
Mini-marshmallows (optional)Cut the sweet potatoes into 1 1/2 - 2 inch thick slices. Dump them into a big pot with water and bring to a boil. Let them simmer until the skin slides off easily. The potatoes themself should not be allowed to get too soft, or they'll be difficult to layer. It usually takes 10-15 minutes to get them to this point.
Remove skin from slices. You'll need some help with a paring knife, but if you get a good batch of potatoes it should slide off pretty nicely with minimal help.
Spray the heck out of your casserole pan with Pam or some other high quality non-stick stuff. Be generous. The dish can be a misery to clean afterwards, and anything you can do to help yourself up front is worth your while.
Put down one layer of sweet potato slices on the bottom of the dish. "Dot" with butter. I can't really tell you how much butter to use, but I strive to get as even coverage as I can. Then sprinkle liberally with brown sugar. Put down a second layer and repeat the butter and brown sugar thing. You'll probably have enough sweet potatoes left to "fill in gaps" but not enough to do a third layer. Finally, if desired, top with mini-marshmallows.
Bake in 375 degree oven for three quarters of an hour, give or take. If the sweet potatoes are soft when you stick them with a fork, then it's done, though you can let it go a bit longer to get a nice carmelized texture to it.
If you want a nice toasted marshmallow top, just before serving throw a few more marshmallows on the top and turn the broiler on. Keep a close eye on the procedings though, so it doesn't burn.