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Adjust an oven rack to the lower-middle position and heat the oven to 450 degrees.
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Spray a 9-inch round cake pan or 8-inch square baking pan with nonstick spray.
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Measure 1/3 cup cornmeal into a medium bowl.
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Whisk the remaining 2/3 cup cornmeal, sugar, salt, baking powder, and baking soda together in a small bowl; set aside.
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Pour the boiling water all at once into the 1/3 cup cornmeal and stir to make a stiff mush.
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Whisk in the buttermilk gradually, breaking up lumps, until smooth, then whisk in the egg and melted butter.
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Stir the dry ingredients into the mush mixture until just moistened and pour the batter into the prepared pan.
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Bake until the bread is golden brown, about 20 minutes.
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Remove from the oven and flip the cornbread onto a wire rack.
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Turn the bread right-side up, cool for 5 minutes, and slice.