Preheat the oven to 350°F. Line an 8x12x1 inch sheet pan or a 9x13-inch baking dish with parchment paper or spray with nonstick cooking spray. Set aside.

Combine the brown sugar and 1 cup of the butter and beat until light and fluffy. Add in 1 teaspoon of the vanilla and the eggs and beat until combined. In another bowl, combine the oats, flour, baking soda and 1/2 teaspoon of the salt. Stir the dry ingredients into the butter/sugar mixture until combined.

Reserve about one-third of the mixture and press the remaining mixture into the prepared pan.

In a small saucepan, combine the sweetened condensed milk, chocolate chips and remaining butter and place over medium-low heat. Stir constantly until melted and smooth. Remove from the heat and stir in the remaining 1 teaspoon of vanilla and 1/2 teaspoon salt. Spread the chocolate mixture over the oat mixture in the pan, then drop the remaining oat mixture by teaspoonfuls over the chocolate mixture.

Bake in the preheated oven until golden, about 25 minutes. Let cool slightly before cutting into bars.