Position a rack in middle of oven and preheat to 375°F. Butter a 2-quart soufflé dish.
Place cornmeal in a large bowl. In a small bowl, whisk baking powder, cayenne and salt. Bring milk just to a simmer; whisk into cornmeal until smooth. Whisk in buttermilk, corn, butter, egg yolks, parsley and baking powder mixture.
Using an electric mixer on medium-high speed, beat egg whites just until they hold stiff peaks. Gently fold egg whites and 1/2 cup Cheddar into cornmeal mixture. Transfer to soufflé dish and sprinkle with remaining 1/2 cup Cheddar.
Bake until top is golden brown and spoon bread is set around edges but still slightly jiggly in center, 35 to 40 minutes. Let stand for 10 minutes before serving.