In a small mixing bowl combine flour and baking powder. Beat oil, sugar, and cocoa together in a large mixing bowl. Add eggs to oil and cocoa mixture one egg at a time, beating well after each addition. Stir in vanilla. Mix in flour mixture. Fold in nuts if desired. Chill the dough overnight. Preheat oven to 350 degrees. Grease cookie sheet. Roll dough into walnut-size balls and roll each ball in confectioners' sugar. Bake cookies for 12 to 15 minutes.