Sweet Potato Spoon Bread

4 Tbsp. unsalted butter, melted
2 medium sweet potatoes (about 1 lb.)
2 1/2 C. milk
1 Tbsp. fresh thyme
1 Tbsp. light brown sugar
2 tsp. coarse sea salt
1/2 tsp. freshly ground black pepper
1 C. finely ground white or yellow cornmeal
4 large eggs, separated
2 tsp. baking powder

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