Preheat oven to 400 degrees F. Core the apples – but not all the way – using an apple corer or a paring knife. You want to cut about 2/3 of the way down to remove the seeds if you don’t have an apple corer. Carefully cut a circle around the stem and cut about an inch downward. From there, use the same knife or a spoon, remove the core and the seeds.
In a bowl, combine oats, brown sugar, flour, salt and cinnamon. Add softened butter and vanilla, and mix thoroughly with a fork and your hands until it becomes clumped together. If desired, you can also add nuts here! Stuff the mixture equally inside each apple. Sprinkle with additional cinnamon if desired.
Place apples in a large baking dish. Pour apple cider and bourbon in the bottom of the pan, then bake for about 45 minutes. This will depend on the exact size of your apples (if using small/medium apples, start by baking for 30 minutes), but it is easy to keep an eye on them because a few times during the cooking process (every 10 minutes or so), you want to spoon the cider/bourbon mixture overtop of each apple. Once the skin looks all wrinkly and slightly darker in color, remove from the oven. Serve immediately with ice cream on top.