Simply make brownies to crumbles, mix it with rum ( I use A LOT), until the consistency is enough to make the dough didn't stick on hands when you shape it like a ball. If you bake your own brownies, do the Rum part during mixing the batter. You can add more rum as the replacement of icing later again when making dough for pop cake.
Extra tips:
Be sure to not putting too much, most people tend to accidentally put too much icing (or Rum, in this case) for pop cake, making it too moist. The right mix is when the dough didn't sticks on your hand when shaping it to a ball. And do use hand instead of spoon. You'll feel the right consistency of the dough when you use hand.
2. Make a round shape flat dough, put some pop rock candy in the middle, than shape it to ball. Open pop rock candy bag just before you use it. Pop rock popping part fades easily to humidity.
3. Stick a pop-sickle stick or lollipop stick to your ball. Drip some melting chocolate around the stick & ball too glue them together.
4. Put in fridge for 5 minutes until the chocolate & the ball harden
5. When it's hard enough, dip balls to melting chocolate.
6. Put in fridge for 5 minutes until the outer chocolate shell harden again. Then you can decorate it with any toppings you like.
Remember to eat it right away. As I said before, pop rock popping part fades easily. The rum will kicks, just without explosions in your mouth. Must be eaten at the same day for the most dramatic effect.