Preheat the oven to 200°F.
Place a wire rack inside a large sheet pan and set aside. This will keep the cooked waffles warm and crisp while you finish the rest.
Melt the butter, then stir in the vegetable oil. This helps bring the melted butter closer to room temperature before adding it to the wet ingredients.
In a large mixing bowl, whisk together the all-purpose flour, fine cornmeal, sugar, baking powder, baking soda, and kosher salt.
In another mixing bowl, or in a large measuring cup with a spout, whisk together the buttermilk, eggs, melted butter, and vegetable oil.
Pour the wet ingredients into the dry ingredients.
Use a large silicone spatula to gently fold everything together until the last streaks of dry ingredients are incorporated.
Do not overmix. A few small lumps are fine.
Let the batter rest for 15 minutes while you preheat your waffle maker.
If your waffle maker recommends it, lightly spritz the top and bottom plates with nonstick spray.
Add the recommended amount of batter to the center of your waffle maker.
Close the lid and cook for 3 to 5 minutes, or until your waffle maker indicates that the waffle is done.
If your waffle maker does not have an indicator, the waffle is usually ready when the steam has mostly decreased and the outside is golden and crisp.
Transfer the cooked waffle to the prepared wire rack and sheet pan.
Keep warm in the oven while you cook the remaining waffles.
Serve warm with whipped salted butter and maple syrup.