Preheat oven to 350 degrees.
Beat butter in large bowl until creamy. Beat in flour, sugar, and salt until crumbly.
Using a spatula or other flat surface, press 2 cups of the crumb mixture into the bottom of a greased 13x9 inch pan. (Reserve remaining mixture.).
Bake for 10 to 12 minutes or until edges are golden brown.
In a small pot, warm 1 1/2 cups of the chocolate chips with the sweetened condensed milk over a low heat, stirring until smooth. Stir in vanilla.
While still warm, spread the chocolate sauce evenly over the hot crust.
Stir the walnuts and a 1/2 cup of the chocolate chips into the remaining crumb mixture. Sprinkle over the chocolate filling. Bake for 25-30 minutes, then cool on a wire rack.
In a small glass bowl, mix 3/4 cup of chocolate chips with the heavy cream. Microwave in 30 second intervals, stirring between each, until mixture is smooth and liquidy (you may need to add more heavy cream if your mixture is too thick).
Using a spoon, drizzle chocolate over the cooling bars.