Preheat oven to 350 degrees F. Butter a 9x13-inch baking dish; set aside.
In a medium bowl, whisk together the flour, baking soda and salt; set aside.
Using an electric mixer, cream together the butter and brown sugar on medium speed until combined and fluffy, about 3 minutes. Reduce the speed to low and slowly add the flour mixture until fully incorporated. Add the oats and mix on low briefly to just combine.
Transfer approximately half of the dough mixture to the baking dish. Spread and press the dough into the baking dish until you have an even layer with no cracks or crumbles. Bake for 10 minutes, then remove from oven.
While the dough is baking, combine the evaporated milk and caramels in a small saucepan over medium-low heat. Stir the mixture constantly until all the caramels are melted and you have a thick sauce. Allow to cool until cookie base comes out of the oven.
Once out of the oven, sprinkle the cookie base evenly with chocolate chips and pecans, and then carefully pour the caramel sauce over the cookie base, nuts, and chocolate. Use an offset spatula to spread the caramel sauce into an even layer.
Sprinkle the remaining cookie dough over the top of the caramel-chocolate-nut mixture a handful at a time, making sure to get an even coating over the whole dish.
Return the dish to the oven, baking until the topping is lightly browned, about 15 minutes.
Once out of the oven, cool the dish to room temperature and then refrigerate at least 2 hours or overnight. Cut into 2-inch squares and serve. Store leftovers in an airtight container at room temperature or in the refrigerator.