Preheat your oven to 375°F (190°C). Generously butter a 1.5-quart soufflé dish or deep casserole dish.
In a medium saucepan, bring the milk to a gentle simmer over medium heat. Slowly whisk in the cornmeal in a steady stream, stirring constantly to prevent lumps.
Continue cooking the cornmeal mixture for 2-3 minutes until it thickens noticeably. Remove from heat and stir in the butter until fully melted and incorporated. Allow it to cool for 10 minutes, stirring occasionally.
Whisk the egg yolks, salt, sugar (if using), and baking powder together in a small bowl until smooth. Slowly add this mixture to the cooled cornmeal base, stirring constantly.
In a clean, dry bowl, beat the egg whites with cream of tartar until stiff peaks form.
Gently fold one-third of the beaten egg whites into the cornmeal mixture to lighten it. Then fold in the remaining egg whites with a spatula using a bottom-to-top motion, maintaining as much air as possible.
Pour the batter into the prepared dish and bake for 40-45 minutes until puffed and golden brown. The center should be set but still have a slight jiggle.
Allow the spoon bread to rest for 5 minutes after baking before serving warm.