Preheat oven to 400 degrees.
In large bowl, combine self-rising cornmeal mix, self-rising flour (sifted, spooned and leveled), sugar (if using), yellow corn kernels, jarred diced jalapeno peppers, chopped white onions (if using), one large egg (beaten), and buttermilk.
Stir until well combined. Set aside.
Grease 9" cast iron skillet (or other bakeware) with vegetable oil. Over low-medium heat on top of stove, heat greased skillet until oil is sizzling hot (but not smoking). Watch closely to prevent burning.
Greased skillet is hot enough when a small drop of batter sizzles and turns brown on the bottom. WARNING: Don't lean face close to skillet because hot oil may pop UP when a drop of batter is added to see if it's hot enough.
Turn heat off. Carefully pour batter into hot greased skillet. Smooth evenly with back of spoon.
Use oven mitt to carefully place skillet into preheated oven. Reduce temperature to 375 degrees.
Bake 20 to 25 minutes or until toothpick or knife inserted in center comes out clean.
Serve hot.