Preheat your oven to 350°F (175°C) and adjust the oven rack to the second level position (just above center). Generously spray a 13 x 9 inch baking pan with cooking spray and set aside.
In a medium bowl, whisk together the all-purpose flour, baking soda, and salt. Set this mixture aside.
In a large mixing bowl, use a hand mixer or stand mixer fitted with the paddle attachment to cream together the unsalted butter and light brown sugar on medium-high speed until well blended and mushy. This should take about 2-3 minutes.
Add the eggs and vanilla extract to the butter-sugar mixture, mixing until combined. Scrape down the sides of the bowl as needed to ensure everything is mixed evenly.
Switch the mixer speed to low and gradually add the whisked dry mixture until just combined. Be careful not to overmix here.
Gently fold in the quick oats using a spoon or spatula. Avoid using the mixer for this step to keep the oats intact.
Spread and press down about 2/3 of the oatmeal cookie base into the prepared baking pan. If the mixture is sticky, use a spatula sprayed with baking spray or your clean hands to help.
In a heavy-bottom saucepan, combine the semi-sweet chocolate chips, sweetened condensed milk, unsalted butter, and vanilla extract. Stir over medium heat until the mixture is smooth and well blended, about 5-7 minutes.
Remove the saucepan from heat and stir in the chopped pecans. Immediately pour this fudge mixture over the oatmeal cookie base, spreading it out evenly with a spatula or spoon.
Take the reserved oatmeal cookie base and break it into small and medium-sized chunks. Sprinkle these chunks over the top of the chocolate pecan fudge layer for a rustic look.
Bake in the preheated oven for 25-30 minutes. The oatmeal cookie chunks should be a light to medium golden brown, and the fudge should have a shiny top.
Once baked, remove from the oven and allow the bars to cool in the pan before cutting them into squares. This will help them set properly.
Notes
For a nut-free version, simply omit the chopped pecans.