BEAT shortening, sugar and vanilla in large bowl with electric mixer at medium speed until light and fluffy. Beat in cooled chocolate and eggs.
STIR together flour, baking powder and salt. Gradually add to shortening mixture alternately with milk. Stir in nuts, if desired. Cover and chill about 3 hours or overnight.
HEAT oven to 350ºF. Coat baking sheet with no-stick cooking spray. Shape dough into 1-inch balls. Roll in powdered sugar to coat completely. Place 2 inches apart on prepared baking sheet.
BAKE 13 to 15 minutes or until cookies are set. Cool on baking sheet 2 minutes. Remove to wire rack to cool completely.