One of my favorite pies. This is popular among Southern black people. To me the best ones use nutmeg instead of cinnamon. Sometimes, a little homemade vanilla ice cream goes well on top.
1 1/2 c Sweet potato; boiled 2 Eggs; separated

3/4 c Brown sugar

1 c Milk

2 T Butter; melted

1 ts Nutmeg

1/2 ts Baking powder

1 pn Salt

Mash the boiled sweet potatoes. Beat the egg yolks with the sugar; add to the potatoes with the milk, melted butter and spice. Beat the egg whites, baking powder and salt. Fold the potato mixture in the well-beaten egg whites. Pour into a pastry-lined pie pan. Bake at 350F until a silver knife inserted in the center comes out clean, about 45 minutes.
Addendum 2/2/2006. I made a big boo-boo above and didn't even pay attention to it for years. That should be only 1c Milk rather than 2c. You will get some pretty soggy pie using that much milk. I did make a refinement to this recipe as well. My wife likes a bit sweeter pie than I do, and she wants the center very firm. The best thing to do to achieve both of these aims is to roaster, rather than boil, the sweet potatoes. That way, some of that moisture is evaporated off and the pie is firmer. I also think that your chances of overcooking the sweet potatoes is reduced by roasting them.