Preheat oven to 350 degrees. Grease or line a 13x9 inch baking dish with parchment paper. Set aside.
In an electric stand mixer, beat butter and brown sugar together until light and fluffy. Mix in eggs and vanilla. Stir in oats, flour, baking soda, and salt until well combined.
Press 2/3rds of mixture into the bottom of the prepared baking dish.
In a microwave safe bowl melt together chocolate chips, sweetened condensed milk, butter and salt. Heat for 1 minute, stir until smooth. Heat at 15 second intervals if it needs more.
Mix in vanilla. Stir until well combined. Pour over base mixture, spread evenly.
Crumble remaining base mixture over the fudge layer.
Bake for 25-30 minutes until set and starting to turn golden brown.
Remove from oven and let cool completely before serving.