Preheat oven to 425°. Melt butter in a 10-inch cast-iron skillet in oven 8 minutes.
Whisk together buttermilk and egg in a large bowl; add melted butter from skillet, whisking until blended. Whisk in cornmeal until smooth. Spoon into hot skillet.
Bake at 425° for 30 minutes or until golden.
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