Place a rack in the center of the oven and preheat the oven to 350 degrees F.
Combine the cornmeal, flour, sugar, baking powder and salt in a large mixing bowl. Set aside.
Arrange the bacon strips flat in a single layer in the bottom of a 12-inch cast-iron skillet over medium-high heat. Cook until rendered and crispy, about 8 minutes. Transfer the bacon to one end of a paper-towel-lined plate to drain (save room for the jalapeno!). Add the jalapeno to the bacon drippings and cook until softened and slightly browned, about 4 minutes. Using a slotted spoon, transfer the jalapeno to the plate next to the bacon. Roughly chop the bacon and return to the plate.
Carefully remove 2 tablespoons of the bacon drippings from the pan, leaving the rest. Keep the pan at the ready for baking the cornbread.
Whisk together the buttermilk, oil and eggs in a medium bowl. Add the buttermilk mixture to the cornmeal mixture and use a rubber spatula to fold the dry ingredients into the wet until completely incorporated. Mix in the Cheddar, bacon and jalapeno.
Using a pastry brush, spread the bacon drippings up the sides and around the bottom of the pan. Heat the pan over medium-low heat until the fat just starts to sizzle. Pour in the cornbread batter, allowing the heat to quickly set the bottom crust.
Put the pan into the oven and bake until golden brown and a wooden skewer inserted into the middle of the cornbread comes out clean, 45 to 50 minutes. Remove from the oven and baste the top crust with the butter. Cool for 20 minutes before serving.