Preheat oven to 400°F. Bake sweet potatoes on middle rack 35 minutes or until tender. Cool slightly then peel. Place sweet potatoes in large bowl.
Bring stock, brown sugar, cinnamon and cayenne pepper to simmer in small saucepan. Add to sweet potatoes along with sour cream. Beat with electric mixer on medium speed until smooth. Beat on medium-high speed until light and fluffy.