Fill a pan about 1/3 with eater and bring to a simmer. Place chocolate in a heat safe bowl, then set the bowl over the simmering water, making sure the bottom of the bowl isn't touching the water.
As chocolate starts to melt, stir or whisk chocolate until no lumps remain and chocolate is smooth and shiny. Remove bowl from heat.
Use a spoon to drizzle a small amount of chocolate into the chocolate mold, then use the back of the spoon to fully coat the inside of the mold with chocolate. Make sure the edges are coated with chocolate all the way to the top.
Place mold in fridge for five minutes, or until set. coat the mold with a second layer of chocolate, and place in the fridge for 5-10 more minutes.
Carefully pop the chocolate out of the molds. It helps to wear gloves while doing this to avoid getting finger prints on the chocolate.
Fill three of the halves with hot cocoa powder, a few mini marshmallows and some sprinkles (optional, see notes)
Warm a plate in the microwave for 1 minute. Set the unfilled chocolate halves, edge side down, on the warm plate. The edges will start to melt right away, remove quickly before they melt too much. You just want them melted enough to seal them to the other half. If you have trouble lifting the half off of the warm plate, gently slide the edge of a knife under the edge to lift it up.
Place the unfilled half over the filled half and gently press together to seal the edges.
Drizzle remaining chocolate over the chocolate spheres and top with sprinkles if desired.
When ready to serve, heat 8 ounces of milk until hot and steaming but not boiling. Place hot chocolate bomb in a mug and pour hot milk over it. The hot chocolate bomb will melt and burst. Stir well to mix the chocolate with the milk.
Notes
When choosing sprinkles, look for the kind made of sugar and cornstarch rather than the waxy, rod shaped sprinkles. The waxy sprinkles give the hot chocolate an off taste that is not pleasant.