You need 1 pound of cooked yam, baked or canned, for each 3 people if there are no mashed potatoes, or 1 pound for each 4 if there are also mashed potatoes.

For each 4 pounds of potatoes. you want
2 tablespoons unsalted butter
Sea salt, to taste
Freshly ground pepper, to taste
1 cup miniature marshmallows or 10 to 12 large marshmallows (cut into quarters)

The only traditional option is to stir in a few tablespoons of brown suger and a little cinnamon or some crushed pineapple in juice.

Heat potatoes through (bake in slow oven (250F or (120C for 20-30 minutes, or in microwave).
Cover top with marshmallows.
Return to oven until marshmallows are slightly melted and a bit brown, no more than 10-12 minutes. if the oven is too hot, the marshmallows will disappear! This step cannot be done in the microwave.