PREHEAT oven to 350° F. Grease 13 x 9-inch baking pan.
BEAT butter in large mixer bowl on medium speed for 30 seconds. Beat in flour, sugar and salt on low speed until crumbly. Press 2 cups of mixture onto bottom of prepared pan. Bake for 12 to 14 minutes or until lightly browned on edges.
COMBINE 1 1/2 cups of morsels and sweetened condensed milk in small saucepan over medium-low heat. Heat until morsels are melted, stirring constantly. Remove from heat; stir in vanilla extract. Pour evenly over hot crust. Add remaining morsels and chopped pecans to the reserved crust mixture; sprinkle over the chocolate layer.
BAKE for 25 to 30 minutes or until center is set. Cool in pan on wire rack. Cut into diamond-shaped pieces.