Preheat oven to 375°F. Grease a 2-quart baking dish with butter.
Warm the milk in a saucepan over medium heat until steaming but not boiling.
Whisk in the cornmeal gradually. Cook, whisking, until thick and smooth, about 2–3 minutes.
Stir in butter, sugar, and salt until melted and combined. Remove from heat and let cool 3–4 minutes.
Whisk baking powder and egg yolks into the warm cornmeal mixture until glossy.
Beat egg whites in a clean bowl to soft peaks.
Fold the whipped whites into the batter gently, leaving a few streaks.
Pour batter into the dish and bake until puffed, golden, and just set, 30–35 minutes.
Spoon into bowls and serve hot with a pat of butter or drizzle of honey.