1. Heat oven to 350°F.
2. Stir together flour and brown sugar in medium bowl; cut in cold butter until crumbly. Stir in 1/2 cup chocolate chips; press mixture on bottom of ungreased 13x9x2-inch baking pan. Bake 15 minutes.
3. Combine sweetened condensed milk, egg and vanilla in large bowl. Stir in remaining 1-1/2 cups chips and nuts. Spread evenly over hot baked crust. Bake 25 minutes or until golden brown. Cool completely in pan on wire rack. Cut into bars. Makes about 24 bars.