Preheat oven to 350 degrees F. Grease 13 x 9-inch baking pan. Beat butter in large mixer bowl until creamy. Beat in flour, sugar, and salt until crumbly. With floured fingers, press 2 cups crumb mixture onto bottom of prepared baking pan; reserve remaining mixture. Bake for 10 to 12 minutes or until edges are golden brown. Combine 1 cup chunks and sweetened condensed milk in small, heavy-duty saucepan. Warm over low heat, stirring until smooth. Stir in vanilla extract. Spread over hot crust. Stir in walnuts and remaining chunks into reserved crumb mixture; sprinkle over chocolate filling. Bake 25 to 30 minutes or until center is set. Cool in pan on wire rack. Cut into bars.