Preheat your oven to 350°F then line a 9x13 inch baking pan with parchment paper and set it aside.
In a large bowl, cream together the softened butter and brown sugar until light and fluffy. Add the egg and vanilla extract and mix until well combined.
Add in the flour, baking soda, salt, and oats and mix until the flour mixture disappears.
Press about two-thirds of the oat mixture into the bottom of the prepared baking pan, creating an even layer.
In a saucepan, combine the sweetened condensed milk, chocolate chips, butter, and salt over low heat. Stir continuously until the chocolate chips and butter have melted and the mixture is smooth. Add in the vanilla extract and mix off the heat.
Pour the fudge filling over the oat base in the baking pan, spreading it out evenly.
Crumble the remaining oat mixture over the top of the fudge layer, and bake for 25 minutes.
Remove from the oven and let the bars cool completely in the pan. Once cooled, cut into squares and serve.