Preheat your oven to 350 degrees. Grease a 10 x 15-inch jelly roll pan with a 1-inch edge.
In a mixing bowl using a hand mixer cream together the brown sugar, melted butter, eggs, and vanilla.
Add in the flour, baking soda, salt, and old-fashioned oats and mix everything thoroughly. With greased hands or a rubber spatula press 2/3 rds of the oatmeal mix on the bottom of the prepared baking sheet. Saving 1/3 rd of the dough for adding to the top later.
In a medium-sized saucepan, over medium heat. Mix the sweetened condensed milk, chocolate chips, butter, and salt. After all the chips are melted and the chocolate mixture becomes smooth remove it from the heat. Carefully add in the vanilla, this is where you have the option to add in the chopped nuts if you so choose. Pour over the oatmeal base and smooth out to the edges.
Drop chunks of the remaining oatmeal dough over the top and gently press it down with your hands to flatten it out slightly.
Bake for 25- 30 minutes. Cool completely before you cut them. These are good for a week in an airtight container or store them in the freezer for up to 3 months. If you stack them in a container for storage layering wax paper between the layers to help them not stick together.