Heat the oven to 350 degrees F (177 degrees C). Generously butter a 6.5-inch cast iron skillet or a 5x5-inch baking dish and set aside.
In a medium-sized bowl, cream together the melted butter, brown sugar, and granulated sugar, until well blended. Beat in the egg yolk and the vanilla.
In a separate small bowl, mix together the flour, oats, baking soda, and ground cinnamon. Pour the flour mixture into the butter/sugar mixture and mix until combined. Gently stir in the chocolate chips.
Press the cookie dough into the skillet and bake for 15-18 minutes or until the top of the cookie looks barely golden. Do not over-bake it, the cookie will continue to cook in the hot skillet when you take it out of the oven.
Allow the cookie to cool in the skillet or baking dish for at least 10 minutes before cutting.
Best when enjoyed warm.
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Notes
Butter Your Skillet Generously: A lavish layer of butter in the skillet is the secret to achieving those crispy, rich edges on your oatmeal chocolate chip skillet cookie. Don't skimp!
Properly Measure the Flour: Incorrectly measured flour can ruin the texture. Instead of scooping, spoon the flour into your measuring cup and overfill it slightly. Level it off using a straight-edged utensil like the back of a knife. This ensures you don't compact the flour, leading to too much flour in the recipe.
Be Mindful of Baking Time and Cooling: Cast iron holds heat remarkably well, meaning your cookie will continue to cook even after it's out of the oven. Aim for a bake time that leaves the top lightly golden, typically around 15-18 minutes. Allow the cookie to cool for at least 10 minutes before diving in. This helps the center set, transforming your cookie into the perfect indulgent treat.