Preheat oven to 350 degrees. Butter a 2-quart baking dish and dust with cornmeal. Steam sweet potatoes until tender, about 15 minutes. Remove from heat and mash until very smooth; let cool slightly.
Meanwhile, in a medium saucepan, bring milk to a simmer. Whisk in cornmeal in a thin stream. Cook, whisking constantly until just thickened, 1 to 3 minutes; remove from heat and let cool slightly.
In a large bowl, stir together mashed potatoes, cornmeal mixture, egg yolks, sugar, molasses, ginger, and salt. Whisk the egg whites until stiff peaks form. Stir in half of egg whites into cornmeal mixture. Very gently fold in remaining egg whites.
Spoon mixture into prepared baking dish, place on a baking sheet and bake until puffed and set, 35 to 40 minutes.