Preheat oven to 400°F. Grease skillet or pan. If using cast-iron, preheat inside oven.
Whisk together cornmeal, flour, sugar, baking powder, and salt in a large bowl.
In another bowl, whisk buttermilk, eggs, and melted butter. Stir in most cheese (and jalapeños if using).
Combine wet and dry gently with a spatula, leaving small lumps.
Pour batter into skillet/pan. Top with reserved cheddar.
Bake 20–25 minutes until golden, edges crisp, and toothpick comes out clean.
Cool 5–10 minutes before slicing.