Preheat oven to 400*F. Generously grease 2-quart souffle' or casserole with 1 tbsp. butter.
Wrap potatoes in foil. Bake 45 to 55 minutes until soft to touch. Remove from oven. Discard foil; cool. When cool enough to handle, remove and discard peels. In large bowl mash potatoes.
Reduce oven to 350*F. In large saucepan, bring milk, thyme, sugar, salt, and pepper to low boil over medium heat. In slow steady stream, whisk cornmeal into milk mixture. Cook, whisking cornmeal, 4 to 5 minutes, until mixture is thick and pulls away from bottom of pan. Remove from heat, cool slightly. Add potatoes, egg yolks, remaining 3 tbsp. butter and baking powder to milk mixture; stir to thoroughly mix.
In large mixing bowl beat egg whites with electric mixture until soft peaks form. Gently fold whites into the potato mixture.
Spoon batter into prepared dish. Bake 35 to 40 minutes, until internal temperature reaches 165*F. Edges will be firm, and center a little soft. Remove from oven, Let stand 10 minutes. Serve warm