Put a buttered 10 inch cast iron skillet in the oven and heat both to 400F. The hot pan is what makes the crust.
Whisk the cornmeal, flour, baking powder, salt and sugar in one bowl; the buttermilk, eggs and melted butter in another.
Fold wet into dry until just combined; streaks of flour are fine. Overmixing toughens cornbread.
Fold in the cheese, corn and jalapeno with as few strokes as possible.
Pour the batter into the hot skillet; it should sizzle at the edges on contact.
Bake 25 to 30 minutes, until golden and a toothpick in the center comes out with moist crumbs, not wet batter.
Cool in the pan 10 minutes before cutting; the crumb finishes setting as it cools.