Heat oven to 350F. Stir together flour and brown sugar in medium bowl. Cut in butter with pastry blender or fork until mixture resembles coarse crumbs. Stir in 1/2 cup peanut butter chips. Press mixture into bottom of ungreased 13x9x2 inch baking pan. Bake 15 minutes.
Meanwhile, in large bowl, combine sweetened condensed milk,egg,and vanilla. Stir in remaining chips and walnuts.Spread evenly over hot baked crust.
Bake 25 minutes or until golden brown. Cool completely in pan on wire rack. Cut into 2" or 2 1/2" squares; cut squares diagonally into triangles. Sift powdered sugar over top, if desired.