Set aside 2 tablespoons of the butter. Beat the remaining butter with mixer on medium speed for 30 seconds. Add brown sugar and beat until well mixed. Beat in eggs and 1 teaspoon vanilla. In another large bowl stir together flour and baking soda then 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 1 teaspoon vanilla.
Press three quarters of the oat mixture into the bottom of an ungreased 15×10x1inch baking pan (or something close to it). Spread chocolate mixture over the oat mixture. Use your fingers and dot the remaining oat mixture over the chocolate. Tip: Wet your fingers slightly to help prevent sticking.
Bake for 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.