Prepare 13x9 pan with liner and non-stick spray. Again, I used foil but it wasn't my favorite. You could also use wax or parchment paper.
Line the bottom of your pan with crackers (my layer was 24).
To make the caramel, add condensed milk, milk, butter and brown sugar to a large saucepan on medium heat. YOU MUST STIR THE ENTIRE TIME. Once combined, increase heat slightly and boil for 5 minutes. Be careful, it's really hot and bubbly at this point but you do have to keep stirring or it will burn (you can see in my photos a few burnt pieces. These were kind of unavoidable. If you get them, don't worry, you can't tell at all in the end ?).
Pour about ⅓ of the caramel over crackers in pan. Add another layer of crackers and cover with another ⅓ of the caramel. Repeat one more time so you have 3 layers of crackers and caramel.
In a microwave-safe bowl, add chocolate chips and microwave for 60 seconds. Stir and continue to microwave in 30-second intervals until fully melted. Here is where I add the vegetable oil. It adds a little more shine and makes the chocolate just a little easier to spread. This step is optional.
Spread all of the chocolate over the top of the caramel and crackers and sprinkle with chopped nuts.
Place in the refrigerator for at least an hour to set chocolate. Cut into bars and enjoy!