Bring a large pot of salted water to a boil, cut yams in half and add to water, boil over med-high heat, uncovered.
When potatoes are tender, remove pot from heat, drain water and peel potatoes. Place in a large mixing bowl.
Add brown sugar and the next 8 ingredients to mixing bowl. Beat on med. speed until well blended, about 2-3 min. Pour into prepared pie crust. Bake for 55 min. or until knife when inserted in center comes out clean. Cool on wire rack 1 hr. Refrigerate 2 hrs. Serve cold with whipped topping.