Preheat the oven to 350˚F. Cream together 1 ½ cups softened butter and brown sugar.
Add the rolled oats, all-purpose flour, baking soda, and salt. Mix well.
Pat 3/4 of the mixture into a greased, rimmed jelly roll pan.
Melt the sweetened condensed milk, semi-sweet chocolate chips, and ½ cup of butter in the microwave for one minute. Stir and continue heating and stirring in 15-second intervals.
Stir in the vanilla.
Spread chocolate mixture over the oatmeal layer.
Dot with the remaining oatmeal mixture.
Bake at 350°F for 22-25 minutes and allow to cool before cutting into squares.