Preheat oven to 350F and grease 9" × 13" pan with cooking spray.
In a large mixing bowl (or the bowl of a standing mixer fitted with the paddle attachment) stir together butter, eggs and brown sugar. Add vanilla and milk, stir until combined.
In a separate large mixing bowl, whisk together flour, baking soda and salt. Gradually add to butter mixture and stir until combined. Stir in the oats until evenly distributed. Spread 3/4 of the mixture into the prepared pan (the remainder will be put on top of the chocolate layer).
To make the chocolate filling, melt sweetened condensed milk, chocolate chips and butter in a saucepan over medium heat, stirring continuously.
Spread over the oat layer.Place the remaining dough on top of the chocolate. I flattened small amounts and placed it on top of the chocolate layer until it was almost covered. Bake for 25 minutes (cover with foil if the top begins to brown too quickly).
Let cool then cut into bars. I’m all about eating cookies while they are hot, but these ones are ever better–and easier to cut–once they’re cool.