1. Bake potatoes at 400° for 1 hour or until tender; cool 10 minutes. Increase oven temperature to 450°.
2. Fit piecrust into a 9-inch pie plate according to package directions; fold edges under, and crimp. Prick bottom and sides of piecrust with a fork.
3. Bake at 450° for 9 to 11 minutes or until lightly browned. Reduce oven temperature to 375°.
4. Cut potatoes in half lengthwise. Scoop out pulp into a medium bowl; mash until smooth. Stir in butter and next 4 ingredients until well combined.
5. Whisk egg yolks until thick and pale. Add sugar, and whisk 1 1/2 minutes or until lemon-yellow color. Stir egg yolk mixture into sweet potato mixture until well blended. Add flour, 1 tsp. at a time, stirring until blended after each addition. Add buttermilk, and stir until combined.
6. Whisk egg whites in a bowl until soft peaks form. Gently fold into sweet potato mixture until blended. Spoon sweet potato mixture into piecrust.
7. Bake at 375° for 35 to 40 minutes or until center is set. Let cool on a wire rack 1 1/2 hours or until completely cool. Serve at room temperature, or cover with plastic wrap, and chill 8 hours. Serve with desired toppings.