1. Heat oven to 350°F.
2. Stir together flour and brown sugar in medium bowl; cut in cold butter until mixture is crumbly. Stir in 1/2 cup peanut butter chips; press mixture on bottom of 13x9x2-inch baking pan. Bake 15 minutes.
3. Meanwhile, in large bowl, combine sweetened condensed milk, egg and vanilla; stir in remaining chips and walnuts. Spread evenly over baked crust.
4. Bake 25 minutes or until golden brown. Cool completely in pan on wire rack. Cut into 2 to 2-1/2 inch squares; cut squares diagonally to form triangles. Sift powdered sugar over top, if desired. 24 to 48 triangles.