Preheat the oven to 350°F and fully line a 9-inch square pan with foil, leaving the ends draped over the edge. To insure the bars do not stick, either use non-stick foil or lightly grease the foil.
To make the bars, cream together the butter and the brown sugar until smooth. Whisk together the flour, oats, baking soda, salt and cinnamon, and then stir into the butter and sugar mixture until well combined. Take half the dough and pat it into the bottom of the pan and bake uncovered for 15 minutes.
Remove the pan from the oven, and pour over the oatmeal cookie base half of the sweetened condensed milk, half of the chocolate chips and half of the pecans. Crumble the remaining oatmeal cookie dough into the pan and then pour over it the rest of the sweetened condensed milk, chocolate chips and pecans.
Return the pan to the oven and bake for 30 minutes or until the edges are set and the bars are lightly browned. If using, sprinkle the flaked sea salt on top immediately after removing from the oven. Cover the pan and place in the refrigerator for at least 4 hours so the bars can continue to harden. To serve, lift the Carmelita bars out of the pan before slicing.