Preheat the oven to 450 degrees F. Butter a 9-by-13-inch baking dish; set aside. Combine 1 cup cornmeal and the sugar in a large bowl. Add the hot water and whisk to combine. Let stand 5 minutes.
Whisk the buttermilk, eggs, baking powder and baking soda in a medium bowl, then whisk into the cornmeal mixture until smooth. Add the remaining 2 cups cornmeal and 2 teaspoons salt and whisk to combine. Stir in the melted butter (the batter will be runny).
Pour the batter into the prepared baking dish and bake until golden brown and a toothpick inserted into the center comes out clean, about 25 minutes. Let cool 30 minutes in the pan, then turn out onto a rack and let cool completely, about 2 hours. Wrap in plastic wrap and let sit at room temperature until ready to use, up to 2 days. Cut into cubes before making Fruit-and-Nut Cornbread Stuffing.