Set aside 2 tablespoons of the butter. Beat the remaining butter with a mixer on medium speed for 30 seconds. Add brown sugar; beat until well mixed. Beat in eggs and vanilla. In another large bowl stir together flour and baking soda; stir in oats. Gradually stir dry mixture into beaten mixture. Set aside.
In a medium saucepan combine the 2 tablespoons butter, the sweetened condensed milk, and chocolate . Cook over low heat until chocolate melts, stirring occasionally. Remove from heat. Stir in vanilla.
Press two-thirds (about 3-1/3 cups) of the oat mixture into the bottom of an ungreased 15×10x1-inch baking pan. Spread chocolate mixture over the oat mixture. Using your fingers, dot remaining oat mixture over the chocolate.
Bake in a 350°F oven about 25 minutes or until top is lightly browned (chocolate mixture will still look moist). Cool on a wire rack. Cut into 2×2 inch inch bars.