1.Using fingers, rub oil evenly over inside surface of 2.25-litre (9-cup) metal pudding steamer.
2.Stir chocolate in medium heatproof bowl over medium saucepan of simmering water until smooth (do not allow water to touch bottom of bowl). Reserve about a tablespoon of chocolate.
3.Pour remaining chocolate into steamer; swirl to coat inside of steamer evenly. Continue swirling until chocolate begins to set and stops flowing around the steamer; try to keep the chocolate a uniform thickness, particularly at the top edge. Freeze about 20 minutes or until chocolate sets completely.
4.Pile coins in centre of 30cm (12-inch) round prepared cake board or plate for treasure. Carefully place steamer with set chocolate over coins; using hot cloth, briefly rub outside of steamer. Chocolate shell will slip from steamer to completely enclose coins.
5.Re-melt reserved chocolate; position Smarties, one at a time, on piñata securing with a tiny dab of chocolate.
6.Tie ribbon around piñata, position hammer on board.
The piñata is quick and easy to make, but make sure the top edge of the shell doesn’t have any weak spots. If the piñata does break, re-melt the chocolate and start again. The piñata can be made several days ahead of the party.Note