PREHEAT oven to 350° F. Grease a 13 x 9-inch baking pan.
BEAT butter in a large mixing 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 morsels and sweetened condensed milk in a small, heavy-duty saucepan. Warm over low heat, stirring until smooth. Stir in vanilla extract. Spread over hot crust.
STIR nuts and remaining morsels into reserved crumb mixture; sprinkle over chocolate filling. Bake for 25 to 30 minutes or until center is set. Cool in pan on wire rack.