Preheat oven to 350°F (175°C). Grease a 9x13 inch baking pan.
In a large mixing bowl, beat 1 cup of softened butter and 2 cups of packed brown sugar until creamy. Add 2 teaspoons of vanilla extract and 2 large eggs, beating well.
In a separate bowl, combine 2 ½ cups of all-purpose flour and 1 teaspoon of baking soda. Gradually add to the butter mixture. Stir in 3 cups of quick-cooking oats.
Press two-thirds of the oat mixture into the bottom of the prepared baking pan.
In a saucepan, combine 1 can of sweetened condensed milk, 2 cups of semi-sweet chocolate chips, and 2 tablespoons of butter. Cook over low heat, stirring until smooth. Remove from heat and stir in 1 teaspoon of vanilla extract and ½ cup of chopped walnuts if using.
Spread the chocolate mixture evenly over the oat mixture in the pan. Drop the remaining oat mixture by spoonfuls over the chocolate layer.
Bake for 25 minutes or until the top is lightly browned. Cool in the pan on a wire rack before cutting into bars.