Line a 13 x 9” baking pan with foil, but do not grease. In large bowl, beat 14 oz butter until smooth. Add brown sugar, beat until well mixed. Beat in eggs and 2 tsp vanilla. Add flour, baking soda and oats and mix well.
In a small saucepan, combine sweetened condensed milk, chocolate, and 2 tbsp. butter. Over low heat cook until melted. Add in 2 tsp vanilla and stir until smooth. Remove from heat.
Press about two thirds of mixture into the bottom of the 13 x 9 pan.Spread melted chocolate mixture over this. Press remaining oat mixture over the chocolate.
Bake at 350 for about 25 minutes. Bars will look golden but still a little melted. Cool on a wire rack.