Bring butter to room temperature. Melt chopped chocolate and chopped copha over hot water, remove and allow to cool without setting. Cream butter and beat in chestnut spread and rum. Stir in cooled chocolate mixture. Melt additional chocolate over hot water (I'd use dark chocolate instead of milk....it turns out much better). Place 36 coloured foil cases on tray and pour 1/2 teaspoon melted chocolate into each case, just enough to cover base. Reserve remaining melted chocolate.

Place chestnut mixture into a piping bag fitted with a fluted tube and pipe swirls of mixture into foil cases. Trickle reserved melted chocolate over chestnut mixture in cases. Keep refrigerated until serving.