Preheat oven to 350 degrees F. Line a 9x13-inch baking dish with a foil or parchment paper sling to make the bars easier to remove after baking.
In a large bowl, beat butter and sugar until light and creamy. Add eggs and vanilla, mixing well to combine.
Add oats, flour, baking soda, and salt. Stir just until combined.
Meanwhile, in a medium saucepan or microwave safe bowl, combine the chocolate chips, sweetened condensed milk, and butter. Cook over medium heat, stirring constantly, until melted and smooth, or in the microwave in 30 second bursts, stirring between each burst, until melted. Stir in the vanilla and salt.
Use your hands to press about ⅔ of the cookie mixture into the bottom of the prepared baking dish, then pour the melted chocolate mixture over the top. Crumble the remaining cookie mixture over the top of the chocolate, leaving spaces where the chocolate can peek through.
Bake for 25-28 minutes just until golden brown on top (don't overbake or the bars won't stay as soft), then cool completely for at least three hours before cutting and serving.
These will keep for 3-4 days at room temperature in an airtight container, or in the fridge for 5-7 days.