In medium bowl, mix 1/2 cup peanut butter, the butter, peanuts and vanilla. Stir in 2 cups powdered sugar, 1/2 cup at a time, until stiff dough forms. (Work in powdered sugar with hands if necessary.) If dough is crumbly, work in additional 1 tablespoon peanut butter. Pat mixture in pan. Cover; refrigerate about 1 hour or until firm. Remove from pan, using foil edges to lift. Cut into 8 rows by 8 rows.