In large bowl, beat butter and remaining 1/4 cup brown sugar with electric mixer on medium speed about 3 minutes or until smooth. Add nut mixture; beat about 1 minute or until mixed. Beat in egg and vanilla. With spoon, stir in flour mixture about 1 minute or just until blended. Shape dough into 2 balls; flatten each ball into a disk. Wrap separately in plastic wrap; refrigerate at least 2 hours until firm.