Preheat oven to 350°F and line a large baking sheet with parchment paper; set aside.

In a medium bowl, stir the flour, cinnamon, baking soda, baking powder, and salt together.

In a large bowl, cream the butter and brown sugar together with a hand mixer for 1-2 minutes. Then, add the milk and vanilla and stir to combine. Pour the dry ingredients into the wet, then stir to combine. Last, use a rubber spatula to fold in the oats and apple.

Use your hands to roll the dough into 18 medium-large balls (NOTE: you will either need to bake these in batches or use two baking sheets) and place them on the baking sheet, making sure they’re spaced two inches apart.

Place the sheet in the oven and bake the cookies for 15-17 minutes or until the edges are crispy but the middle is still slightly underdone. Leave the cookies on the sheet for 5 minutes before carefully transferring to a wire rack. Sprinkle with flakey sea salt before serving and enjoy!