First, center a rack in your oven and preheat it to 400°F. The key here is to ensure that your oven is fully preheated, as this will help the cornbread rise properly. Next, spray an 8×8-inch baking pan with nonstick cooking spray and set it aside.
In a large bowl, whisk together the all-purpose flour, coarse cornmeal, granulated sugar, baking powder, baking soda, and kosher salt. The mixture should be well-combined, with no lumps. Once everything is mixed, gently fold in the shredded cheddar cheese and diced jalapeños, distributing them evenly throughout the dry ingredients.
In a medium bowl, whisk together the buttermilk, melted butter, oil, and eggs. Ensure that the buttermilk is at room temperature, as this will help the batter mix in better. Once combined, carefully pour the liquid ingredients over the dry ingredients, stirring until just combined. It's okay if the batter is lumpy; overmixing can lead to dense cornbread.
Next, pour the batter evenly into your prepared baking pan. The mixture should be thick, but smooth enough to spread out. Use a spatula to help smooth the top if needed, ensuring it's evenly distributed.
Now, place the pan in the oven and bake for 25 to 30 minutes. Keep an eye on it toward the end; you're looking for a golden brown top and a toothpick inserted into the center to come out clean. If it’s still gooey, give it a few more minutes.
Once baked, remove the cornbread from the oven and let it cool in the pan for about 15 minutes. This cooling time allows the cornbread to set and makes it easier to cut. After cooling, cut it into squares and serve warm.
