Step 1

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.

Step 2

In a medium-sized bowl, cream together the melted butter, granulated sugar, and brown sugar until well blended. Beat in the egg yolk and the vanilla.

Step 3

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.

Step 4

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.

Step 5

Allow the cookie to cool in the skillet or baking dish for at least 10 minutes before cutting.

Step 6

Best when enjoyed warm.