Preheat oven to 350 degrees F (175 degrees C). Lightly grease a 9x9 inch square baking pan with parchment paper.
In a large bowl, beat together the butter and brown sugar until fluffy. Mix in eggs and 2 tsp vanilla extract.
In a separate bowl, whisk oats, flour, baking soda, and salt; combine into butter mixture. Set aside.
Using a saucepan over low heat, heat sweetened evaporated milk, chocolate chips, butter, and salt, stirring until smooth and thickens. Remove from heat. Stir in walnuts and 2 teaspoons vanilla.
Put ⅔ of the oat mixture into the bottom of the prepared pan. Spread chocolate mixture evenly over the top, and scatter the remaining oat mixture on top.
Bake for 30-35 mins.Let it cool for 5 minutes then transfer into a wire rack to cool completely. Cut into bars and serve or store in an airtight container for up to