Preheat the oven to 450°. Oil a 9-by-13-inch baking dish and put it in the oven.
In a bowl, combine 1 1/3 cups of cornmeal with the sugar, baking powder, salt and baking soda.
Put the remaining 2/3 cup of cornmeal in a medium heatproof bowl, add the boiling water and stir well. Stir in the buttermilk and eggs, then add the dry ingredients and the melted butter. Mix until just blended. Pour the batter into the hot baking dish and bake for 20 minutes, or until lightly browned. Turn the corn bread out onto a rack to cool completely.
The corn bread can be stored at room temperature for up to 2 days or frozen for up to 1 month.