Heat oven to 350 degrees. In medium bowl, combine flour and sugar; cut in margarine until crumbly. Stir in 1/2 cup chips; press firmly on bottom of 13 x 9-inch baking pan. Bake 15 minutes. Meanwhile, in same bowl, combine sweetened condensed milk, egg and vanilla; stir in remaining 1-1/2 cups chips and nuts. Spread evenly over prepared crust. Bake 20 to 25 minutes or until light golden brown. Cool completely. Cut into bars. Store covered at room temperature.