In medium sized bowl, combine cornmeal mix, flour, corn muffin mix, and sugar (if using), eggs, and buttermilk. Stir until smooth and pourable. Small lumps okay. Set aside.
Grease a CAST IRON fluted bundt pan with vegetable oil (better than cooking spray). Place in hot oven for 5 minutes to warm oil.
Grease or spray a 6-cup ALUMINUM bundt pan with vegetable oil. Place in hot oven for 5 minutes to warm oil.
Wear an oven mitt or use a thick towel to remove HOT bundt pan from HOT oven. Stir batter. Pour (or spoon) batter into bundt pan. It will not fill pan.
Reduce oven to 325 for CAST IRON fluted cake pan. Bake 30 to 35 minutes OR until knife inserted in center comes out clean and cornbread is golden brown on top. See recipe notes for more helpful baking in cast iron tips.
Reduce oven to 350 for ALUMINUM bundt pan. Bake 25 to 30 minutes OR until knife inserted in center comes out clean and cornbread is golden brown on top.
Rest 10 minutes before turning out on a plate.
Superb slathered with butter, honey, or maple syrup.