Preheat oven to 350 degrees F (175 degrees C). Grease an oven-proof cast iron Dutch oven or a deep baking dish.

Heat milk and cornmeal in the top of a double boiler over simmering water, stirring frequently, until thick and creamy, about 5 minutes. Remove cornmeal mixture from heat and stir in butter and 1 teaspoon salt until well combined.

Beat egg yolks, sugar, and 1/2 teaspoon cream of tartar together in a bowl until smooth; stir into cornmeal mixture.

Beat egg whites, 1/8 teaspoon cream of tartar, and 1 pinch salt together in a separate bowl using an electric mixer until soft peaks form. Lift your beater or whisk straight up: the egg whites will form soft mounds rather than a sharp peak. Gently fold egg white mixture into cornmeal mixture until well mixed and fluffy; pour into prepared Dutch oven.

Bake in the preheated oven until golden brown with a moist center, about 30 minutes. Allow to rest for 10 to 15 minutes before serving.