Position a rack in middle of oven and preheat to 375F. Butter a 2-quart souffl 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 souffl 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.