Blend together butter and sugar. Add egg, salt, and vanilla and mix. Stir in flour and mix well. Chill for 1 hour. Mix powdered sugar and crushed peppermint (crush it to almost a powder) and set aside. Form dough into 1-inch balls and bake at 375° on a lightly greased cookie sheet for 7-10 minutes. Roll warm cookies in peppermint mixture.