In a food processor combine walnuts, vanilla wafers and 1 cup of the powdered sugar; pulse several times to mix well. Add rum, a little at a time, just until moistened and it forms a soft pliable dough. Turn out dough onto work surface lightly dusted with confectioners' sugar. Dust your hands lightly with confectioners' sugar, pull off walnut-sized pieces and roll them between your palms to form compact balls. Roll again in confectioner's sugar to coat.