With a hand mixer or stand mixer, beat the butter, shortening, brown sugar, and granulated sugar in a large bowl until well-combined.
Mix in the eggs, molasses, and vanilla extract until combined.
Stir in the flour, oats, baking soda, and salt just until combined. Don't overmix.
Mix in the chocolate chunks.
Cover the bowl, and chill the cookie dough in the fridge for 1 hour.
Preheat the oven to 350F, and line a baking sheet with parchment paper. Set aside.
Using a large cookie scoop (3 tablespoons), scoop out 16 balls of cookie dough.
Place the dough at least 3 inches apart on the prepared baking sheet.
Bake in the preheated oven for about 12-14 minutes, or until the cookies are done. The cookies should look soft and slightly underdone in the middle, but the edges should be set.
Let the cookies cool down on the baking sheet for 10 minutes before transferring to a cooling rack.
Notes
Shortening: The shortening helps the cookies to hold their shape. Using all butter will give you flatter, crispier cookies.
Butter: I prefer to use salted butter in this recipe. If you would like to use unsalted butter, just increase the amount of salt by 1/4 teaspoon.
Light brown sugar: Dark brown sugar will also work.
Flour: Correctly measuring the ingredients is key! I suggest either weighing the flour or aerating the flour (sifting or fluffing) and then lightly spooning it into the measuring cup and leveling. Packing in too much flour will cause the cookies not to spread.