Place the chocolate in a microwave-safe bowl and use bursts of 30 seconds on the microwave and stir in between until completely melted. Or you may use the double boiler method on the stovetop.
Now scoop in a teaspoon or two of chocolate, it doesn't have to be exact into each mold cavity. You can tilt and turn the mold so the chocolate gets on the side or use the back of your spoon to "paint" it up the sides. You can also use a silicone basting brush if you like, or a new paintbrush.If the chocolate is super runny, it made pool up on the bottom which will make that part thicker. To prevent that, flip the mold over onto a piece of parchment paper on a sheet pan.Then place it in the fridge for about 10 minutes.
Take the mold out of the fridge and gently tug the sides of the mold and push underneath the half-ball to pop it out. Use clean hands or wear gloves, especially if you don't want fingerprints on the chocolate bombs.
Fill half of them up with about 1 tablespoon of instant hot cocoa mix. This doesn't have to be exact. You can also add crushed-up peppermint candies or other favorite hot chocolate additions.Then place 5 or 6 mini marshmallows on top of the mix. Don't add too many, if you do the top piece won't stay on very well.
Now take a skillet and put it on a burner at medium heat for just about 45 seconds or so, so it gets really warm, but not super hot, you should be able to still touch the pan. Some people heat up a plate in the microwave as well.
Place the other half-ball of chocolate rim side down onto the pan. The heat will melt the rim evenly and create "glue". Pick it up and place it on the other half that is filled.
If you want, you can pipe chocolate around the seam. This will cover up any gaps if you have them. Smooth out the chocolate with your finger or brush. Or gently roll it on the warm skillet/plate.
Next, if desired, melt and drizzle some white chocolate/candy melts over the top. You can top with sprinkles too if you like. Or like crushed up peppermint candies/candy canes.Once done, place them in mini muffin paper cups for presenting.
Heat up some milk until it is really hot, and then place a hot chocolate bomb in a mug. Pour the hot milk over it and watch as it melts and breaks apart allowing the marshmallows to come to the top. Stir and drink. Enjoy!!