Position a rack in the center of the oven and place a 10- to 11-inch cast-iron skillet or a 9-inch-square metal baking pan on the rack. Heat the oven to 450ºF.
Bring a small kettle of water to a boil. While the water comes to a boil, put 1/2 cup of the cornmeal in a medium mixing bowl. In a small bowl, combine the remaining 1 cup cornmeal with the sugar, baking powder, salt, and baking soda.
Pour 1/2 cup of the boiling water into the 1/2 cup of cornmeal and stir until the cornmeal becomes a thick mush. Whisk the buttermilk into the mush until smooth, then whisk in the egg and egg yolk.
When the oven and pan are fully heated, stir the dry ingredients into the buttermilk mixture. Remove the hot pan from the oven and add the butter or bacon fat, carefully swirling it around the pan until it’s melted and has coated the pan—it’s fine if the butter browns. Immediately pour the excess butter or fat into the batter, stir to combine, then scrape the batter into the skillet or pan.
Bake until the cornbread has pulled away from the sides of the pan and is golden on top, 18 to 20 minutes. Immediately turn the cornbread out onto a cooling rack. Cool for 5 to 10 minutes before serving, or cool completely if using for stuffing.