Position a rack in the lower third of the oven and preheat the oven to 375 degrees F. Lightly butter a 1 1/2 quart baking dish and dust with cornmeal; set aside.
Heat the butter and half-and-half in a medium saucepan over medium-high heat until the butter melts and the half-and-half is steaming. Add 3/4 teaspoon salt. Whisking constantly, slowly pour in the cornmeal. Lower the heat to maintain a simmer and continue to cook, whisking, until thickened, about 3 minutes. Transfer the cornmeal mixture to a large bowl and let cool slightly. Whisk in the egg yolks, one at a time. Whisk in the scallion, baking powder and cayenne.
In a medium bowl, whip the egg whites with a whisk to soft peaks. Fold about one-quarter of the whites into the cornmeal mixture to lighten it, then fold in the rest until no white streaks remain. Pour the batter into the prepared dish and bake until puffed and golden brown, 45 to 50 minutes. Sprinkle with scallion and serve hot.
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