Preheat oven to 350 degrees Fahrenheit. Spray a 9x13-inch baking dish with nonstick cooking spray.

Combine brown sugar and butter; beat until light and fluffy. Add the vanilla and eggs; beat until combined.

In a separate bowl, combine the oats, flour, baking soda, and salt. Add to the butter/sugar mixture until combined.

Reserve about ⅓ of the mixture and press the remaining mixture into the prepared pan.

In a small saucepan, combine the sweetened condensed milk, chocolate chips, and butter over medium-low heat. Stir constantly until smooth, then add the vanilla and salt; stir thoroughly. Spread the chocolate mixture over the oat layer in the pan, then drop the remaining oat mixture over the fudge layer with a spoon (alternately sprinkle the remaining oat mixture like a streusel on top).

Bake for 23 to 25 minutes or until golden. Cool slightly before cutting into bars.