Cream Butter and Sugars: In a large mixing bowl, combine the softened butter, light brown sugar, and granulated sugar. Beat on medium speed for 2 to 3 minutes until the mixture is light and fluffy.
Add Egg and Vanilla: Add the room-temperature egg and vanilla extract to the butter mixture, beating until fully combined, about 1 minute. Scrape down the sides of the bowl as needed.
Mix Dry Ingredients: In a separate bowl, whisk together the oats, flour, cinnamon, baking soda, and kosher salt until evenly distributed.
Combine Wet and Dry: Add the dry ingredient mixture to the wet ingredients, folding gently with a spatula until just combined. Do not overmix.
Fold in Chocolate: Carefully fold the semi-sweet chocolate chunks into the dough until evenly distributed throughout.
Preheat Oven: Preheat your oven to 350°F (175°C) and line two baking sheets with parchment paper.
Scoop Dough: Using a cookie scoop or tablespoon, drop rounded portions of dough onto the prepared baking sheets, spacing them about 2 inches apart.
Add Salt: Sprinkle a tiny pinch of flaky sea salt on top of each cookie before baking.
Bake: Bake for 12 to 14 minutes, until the edges are light golden brown but the centers still look slightly underdone. This creates that perfect chewy texture.
Cool: Allow cookies to cool on the baking sheet for 5 minutes, then transfer to a wire rack to cool completely.