Bake sweetpotato in center of 325˚F oven until tender, about 1 hour. Peel and purée the baked sweetpotato until it is smooth. Set aside.
Butter a 9 X 13 shallow baking dish. Heat milk, half & half, salt and sugar in a heavy saucepan over medium heat. When small bubbles appear around the edge, slowly whisk in cornmeal and flour, stirring constantly. Continue to stir as mixture cooks and thickens. When smooth and creamy, remove from heat and stir in butter until completely melted and combined.
In a medium bowl, beat yolks and cream together until lightened in color. Gradually add to cornmeal mixture. Add 3/4 cup sweetpotato puree. Add salt to taste, if desired. Recipe can be made ahead to this point.
Preheat oven to 350˚F. Beat egg whites and fold into cornmeal mixture. Spread into prepared dish. Bake until puffy, golden brown and just set, about 30 to 35 minutes.