Step 1

1 Position a rack in the middle of the oven and preheat to 350 degrees

Step 2

2 Lightly grease the bottom of a 10-inch cast-iron or other ovenproof skillet with butter

Step 3

3 In a large bowl, combine the 3 tablespoons of butter, the oil, sugar, egg and vanilla and, using a whisk, beat until the mixture is creamy and well combined, about 3 minutes

Step 4

4 (Alternatively, you can use a stand mixer to do this

Step 5

5 )

Step 6

6 In a medium bowl, whisk together the flour, baking soda, cinnamon and salt until combined

Step 7

7 Add the dry ingredients to the wet and stir just to combine

Step 8

8 Stir in the oats and chocolate chips

Step 9

9 Transfer the batter to the skillet, distributing it evenly across the bottom

Step 10

10 Bake for about 15 minutes, or until the cookie is browned lightly around the edges and just set in the center

Step 11

11 Transfer the pan to a wire rack and let cool for 5 minutes before slicing into 10 wedges and serving