Place chocolate in a medium sized microwave safe bowl for 30 seconds. Remove and stir the chocolate for at least 1 minute. The residual heat from the warm chocolate will melt most of the other chocolate in the bowl. If needed, put back in microwave for 15 second intervals, stirring between each one.
Pour about a teaspoon of melted chocolate into the mold. Assuming your mold is 2 inches. You'll need more chocolate if using 2 ½ inch molds and even more chocolate for 3 inch molds. Use the back of the spoon to cover the entire half circle with chocolate, making sure to go all the way up the mold with chocolate. Place in fridge or freezer to harden. The chocolate hardens fairly quickly, so it should be ready for the second coat in less than 5 minutes.
Repeat the process with a second coat of chocolate.
Pull the mold out of freezer again and make sure it is fully hardened. Place a teaspoon or so of hot cocoa mix in each side of the mold. Or place hot cocoa mix in one side and extra marshmallows in the other. Honestly, try to get at least 1 ½ to 2 teaspoons of mix inside each bomb or you may have to use two bombs per drink! Most mugs hold more than 1 cup of hot milk.
Make sure you have melted chocolate to seal the bombs. Take one half of the bomb and quickly place it on top of the other half. If you need to you can warm a plate under hot water for a few minutes until almost too hot to handle. Then press chocolate half onto that dry, hot plate for a few seconds to harden and then press to seal on the other half.
With a spoon full of melted chocolate, run it around the seam of the bomb. Use your finger to smooth it out and fill in any cracks from uneven edges. Add sprinkles or crushed candy bars to the wet chocolate and allow the seal to harden.
Now for the fun part - time to use your hot chocolate bomb! Warm a glass of milk in the microwave or on the stove top. You want it hot, but not boiling.
Place hot chocolate bomb in the glass and stir. The hot milk will make your bomb open up and release the hot cocoa mix. Stir to combine the loose hot cocoa and marshmallows. Allow to cool slightly and enjoy!