Preheat oven to 425ºF. Lightly grease an 8-inch cast iron skillet or baking dish.
In a large bowl, combine the dry ingredients: yellow cornmeal, all purpose flour, shredded cheddar cheese, diced jalapeno, sugar, baking powder, baking soda, and kosher salt.
In a separate bowl, whisk the wet ingredients: buttermilk, melted butter, and egg.
Fold the wet ingredients into the dry until just combined. Be careful not to overmix.
Pour batter into the prepared skillet, spreading evenly.
Bake 20-25 minutes, or until the edges are crispy, the top is golden brown, and a toothpick comes out clean.
Remove and allow to cool slightly before slicing and serving.