Pour candy melts into a microwave safe bowl and melt for 30 seconds, stir, and repeat.
In a small saucepan bring cream to a light simmer. Add chocolate and remove from heat. Stir until all chocolate has melted. Whisk in rum; taste and adjust rum levels if needed.
Pour a dollop of melted dark chocolate into each mold cavity. Using the paintbrush, guide chocolate up sides to cover cavity. Pop mold into the freezer for a couple of minutes.
Remove, and pour a little bit of rum chocolate into the cavity - not too much! You need to leave space at the top for the candy bottom.
Spoon dark chocolate over the top of each cavity, making sure to completely cover the filling. Once filled, gently tap mold on the counter to even out bottoms and release any trapped air bubbles.
Put in freezer for 10-15 minutes before removing candies from mold by turning upside-down and tapping on the counter. Decorate with a paintbrush dipped in lustre dust, if desired.