In a small bowl, beat butter and cream cheese until smooth; stir in flour. Shape into 1-in. balls. Press dough onto the bottom and up the sides of ungreased miniature muffin cups. Spoon cashews into shells; set aside.
In another small bowl, beat the brown sugar, egg and vanilla until combined; spoon over nuts. Bake at 350° for 20-25 minutes or until filling is set and pastry is golden brown. Cool for 10 minute before removing from pans to wire racks.