Add 5 ounces of chocolate to a medium bowl and microwave for 30 seconds, remove and stir fully and scraping down the sides of the bowl with a rubber spatula, then continue to microwave at 15-second intervals, stirring and scraping between each one, until almost completely melted. Continue to stir so that the heat of the chocolate melts the unmelted pieces. Check the temperature to make sure that the chocolate reads between 82 and 84 degrees F. If it goes over 84 degrees, stir in an additional ounce of chocolate until melted to bring it back down to 82 to 84 degrees. If you are using two molds, you can do 10 ounces then add in 2 ounces, if needed. You don’t want that chocolate to sit too long and have to be reheated again if you only have one mold. If you do not temper the chocolate with a thermometer and this process, the chocolate can turn out chalky or discolored instead of shiny.
Spoon about 1 tablespoon of melted chocolate into each mold and use the back of the spoon to spread it around, make sure to coat as evenly as possible and taking special care around the top and not leave any exposed areas. Place the mold on the small baking sheet and refrigerate for 5 minutes.
Remove from the refrigerator and spoon another heaping spoonful into the molds one at a time - do not add chocolate to each cavity at once because it will start to cool too quickly and become thick, gritty, and hard to work with. Use the back of the spoon to work the second layer of chocolate around making sure to build up the rim, you want a slightly thicker edge to ensure they don’t crack. Freeze for 10 minutes.
Remove from the freezer and put on the food-safe gloves and gently remove the chocolate shells from the molds. Repeat steps 1 through 4 if using a single mold.
Once all of your shells are made, microwave a flat plate in the microwave for 2 minutes, then place 6 of the 12 halves on the plate one at a time using a gloved hand to gently spin them to smooth the edges then place round side down in a standard-sized cupcake liner on your work surface. Wipe the plate off after all 6 are done.
Fill each of the half spheres in the cupcake liners with a packet of hot cocoa mix and marshmallows if desired.
Reheat the plate for 2 minutes in the microwave. Then repeat the process with the other 6 halves and place them on top of the cocoa filled halves and gently press to seal.
Let the hot chocolate bombs set for a few minutes then top with a drizzle of chocolate and sprinkles if desired.
To make hot chocolate, heat the milk in a small saucepan over medium heat just until the edges begin to bubble - the ideal temperature for the milk is 160°F - then remove and pour over a hot chocolate bomb in a large mug and stir until fully mixed.