Preheat the oven to 400°F and place the rack in the middle position.
Once the oven is preheated, add the butter to a 10-inch cast iron skillet, place in the oven and heat until melted, about 10 minutes.
Whisk together cornmeal mix, sugar, and salt in a medium bowl.
In a large measuring cup or medium bowl, whisk together the eggs and buttermilk, then stir the wet ingredients into the dry ingredients.
Once the butter is melted and hot, remove the pan from the oven, carefully swirl the butter around the bottom and sides, then pour the remaining butter into the batter. Stir the butter in, then pour it into the hot skillet.
Return the cornbread to the oven and bake until the top is light golden brown and an inserted toothpick comes out mostly clean, about 18-22 minutes. Do not overbake.
Remove the cornbread from the oven and let cool 10 minutes in the pan. Carefully release from the pan, cut into slices or squares and serve warm or room temperature.