Melt the chocolate in the microwave starting with 30 seconds and then moving to 15 second increments, stirring in between and checking the temperature. All the chocolate should be melted and no more than 90° (see notes for types of chocolate and temperatures)
Place some melted chocolate in the mold and work it around the half sphere and up the sides. Be sure to get good coverage. You may need a second coat after it sets (see post for details).
Let the chocolate set up either at room temperature (about 10 minutes) or speed it up by placing it in the freezer or refrigerator for 5 minutes.
Remove the set chocolate from the molds. Fill half of the half-spheres with 1 tablespoon of hot cocoa mix and as many mini marshmallows that will fit.
Take the remaining half-spheres, one at a time and gently melt the edge on a plate that has been heated in the microwave for 1 minute. Then place it over a filled mold to seal.
After all the molds are sealed and set, you can decorate by drizzling additional chocolate and sprinkles.
To serve, place the bomb in a mug. Pour 8-12 ounces (depending on the size of your mug) hot milk over it, stir. ENJOY!