Place the unpeeled yams in a large pot, cover with cold water and bring to a boil over medium-high heat.
Boil for 15 to 30 minutes, or until the yams are cooked through and tender.
Drain and let cool somewhat.
Peel the yams, chop them into large pieces and place them into a large bowl with the butter, salt and pepper.
Mash with a potato masher until very smooth.
Alternatively, put the yams through a potato ricer and then mix with the butter, salt and pepper.
Place the fufu into a large serving bowl. Wet your hands with water, form into a large ball and serve.