The root vegetables sold in the US as "yams" are actually a variety of sweet potato. True yams (genus Dioscorea) are monocots (e.g., lilies, onions, agaves) native to Africa. Sweet potatoes (genus Ipomoea, including the "yams" sold in the US) are dicots native to the Americas and are in the same order as solanaceous plants (family Solanaceae, which includes tomatoes, eggplants, and potatoes). Suffice to say that true yams and sweet potatoes are different (and sadly I've never tasted a true yam, so I can't tell you the culinary difference).
Even though US "yams" and sweet potatoes are just different varieties of the same plant, they're not identical vegetables. "Yams" have a relatively sweet orange flesh that is often a bit fibrous, while sweet potatoes have firmer, mealier yellow flesh that is closer in texture to regular potatoes (though, as the name implies, the sweet potato's flesh is sweeter than regular potatoes). We typically differentiate the two in stores by looking at the color of the skin - "yams" nearly always have a dark skin (often dark red or orange) whereas sweet potatoes have tan skin.
A typical US "yam" (on the left) and sweet potato (on the right) prepared for baking.
I personally prefer using sweet potatoes in this recipe instead of "yams", as I find that the taste of the sweet potatoes goes better with the very sweet topping. We made this for Thanksgiving, so it's this week's end-of-the-week recipe blogging post.
3 pounds sweet potatoes
8 tablespoons (one stick) butter
3/4 cup packed brown sugar
8 ounces marshmallows (approximately)
0. Preheat the oven to 400F.
1. Wash the sweet potatoes (leaving the skin on), and then cut off any blemishes or bad spots. Stab each potato multiple times with a knife to prevent explosions in the oven.
2. Place the sweet potatoes on a baking sheet (lining the baking sheet with foil can make cleanup easier, as the sweet potatoes sometimes ooze a bit of syrup). Bake at 400F until a knife or fork easily pierces the sweet potatoes. This will likely be about an hour, though it depends on the size and shape of the potatoes.
3. Remove the potatoes from the oven and let cool until they can be handled comfortably (at least 20 minutes).
4. Remove the skin (including the hardened layer that forms wherever you removed the skin in step 1). The skin sometimes peels off nicely by hand; otherwise cut off the skin with a small knife.
5. Slice the sweet potatoes into approximately one- to two-inch thick rounds (or whatever size you want) and arrange in a baking dish. Preheat the oven to 350F.
6. Cut the butter into small pieces and distribute on top of the sweet potatoes.
7. Sprinkle the brown sugar on top of the sweet potatoes.
8. Bake at 350F for 40 minutes, sprinkling the marshmallows on top about 20 minutes into baking.
The amounts in this recipe are variable - use whatever proportions suit your tastes. We settled on three pounds of sweet potatoes because it was the amount that fit into our baking dish easily. If you want the marshmallows to be more whole (and less melted), add them later in the second baking (e.g., add them with five or ten minutes remaining).
See here for a picture of this dish (the marshmallows in the picture were only cooked for 15 minutes).