Preheat the oven to 350ºF. Coat a 9x13 inch baking pan with cooking spray.
In the bowl of a stand mixer fitted with the paddle attachment, beat together ¾ cup butter and brown sugar until creamy. Add the egg, yolk, and 1 ½ teaspoons vanilla and beat again.
In a medium bowl, combine flour, oats, ¼ cup nuts, baking soda, and ¾ teaspoon salt. Add to the egg mixture in two or three parts, beating just until incorporated each time.
In a medium saucepan, combine the chocolate chips, sweetened condensed milk, and remaining 2 tablespoons of butter. Stir over low heat until smooth. Stir in ¾ teaspoon vanilla and remaining ¾ cup nuts. Remove from the heat.
Pat two thirds of the oat mixture into the prepared baking pan so that it covers the bottom. Pour the chocolate mixture over the top and spread it out evenly. Take the remaining dough and dot it over the chocolate. Bake 25 minutes or until the cookie top is just beginning to brown. Let cool completely in the pan before cutting and serving.