Break up the chocolate bar and place in a small bowl. Microwave on high for 15-30 second intervals, pausing to stir each time until fully melted and smooth.
Prepare a small baking pan by lining with parchment paper. Set aside.
Place the silicone mold on top of the second (non-lined) baking pan.) Working quickly, fill each cavity with about a tablespoon of melted chocolate.
Lift the mold and swirl it carefully to make sure that the chocolate coats the entire interior of each sphere all the way to the lip. (See notes for suggestions on how to make this step easier.) Invert the silicone mold to allow the excess chocolate to drip off over the parchment-lined baking pan. If desired, you can place the parchment in the freezer until solid and scrape this chocolate back into the bowl to use later.
Set the coated mold on back on the non-lined pan and place in the freezer for about 5 minutes.
When the chocolate has set, remove from the freezer and repeat the process adding another tablespoon of chocolate into each semi-sphere, swirling to coat as much as possible all the way up to the lip. (You will need to do this quickly as the melted chocolate will begin to harden once it touches the cold, set chocolate in the mold.) Return to the freezer for another 5 minutes.
Once set, remove the mold from the freezer. Run your hands under very cold water or hold a few pieces of ice for a few seconds to make your fingers cold; dry hands. Carefully remove each half sphere from the mold, placing them gently on the baking pan.
Fill 3 of the molds with 1 tablespoon of hot cocoa mix and 1 teaspoon of mini marshmallows. Add additional fillings if desired.
Heat a small plate in the microwave for about 15-20 seconds or until warm to the touch.
Chill hands again and dry. Carefully lift a chocolate sphere (these will be the "lids") and place gently on the warm plate for just a second or two until the bottom becomes melty. Quickly place on top of a cocoa-filled sphere half. Repeat with remaining sphere halves. You will have 3 spheres. See notes for tips on how to fully seal the edges if you have gaps.
Repeat this process a second time to create 3 more hot chocolate bombs.
Decorate as desired with a chocolate drizzle, sprinkles, etc.
Gently transfer to an airtight container and keep refrigerated until ready to package or use.