Center rack in the oven and preheat the oven to 350 degrees. Butter a 9X13 inch pan and place the pan on a baking sheet.
Whisk together the flour, baking soda, salt and cinnamon.
Using an electric mixer, beat the butter on medium speed until it is soft and creamy. Add the brown sugar and beat for 2 minutes, then add the eggs one at a time, beating for a minute after each egg goes inches Beat in the vanilla. The mixture should be light and fluffy. Reduce the mixer speed to low and add the dry ingredients, mixing just until they disappear. Still on low speed, stir in the oats and 1 cup of the chopped walnuts.
Set aside 1 1/2 to 2 cups of the mixture, then turn the remaining dough into the prepared 9X13 pan. Set aside while you prepare the next layer.
Set a heat proof bowl over a saucepan of simmering water. Put the condensed milk, chocolate chips, butter and salt in the bowl and stir occasionally until the milk is warm and stir in the vanilla and 3/4 cup walnuts.
Pour the warm chocolate over the oatmeal crust, then scatter the remaining oatmeal mixture over the top. Don't try to spread the oatmeal, and don't worry about getting the topping even. It will create a a pretty open lacy pattern as it bakes.
Bake for 25 to 30 minutes, or until the topping is golden brown and the chocolate layer is dull and starting to come away from the sides of the pan. Transfer the baking pan to a cooling rack for about 2 hours.
Run a blunt knife between the edges of the cookie and the pan and carefully turn the cookie out onto a rack. Turn right side up, then refrigerate for at least 1 hour before cutting. ( Ok so there was no way my fam could wait that long. I was able to cool it for 2 hours but that was it. It was fabulous a bit warm and also later when it was refrigerated!).