Melt the chocolate. Slice a chocolate bar into pieces, or use chocolate chips. Begin by putting plenty of water in the bottom of your double boiler. When the water is boiling, add the chocolate chips or pieces of chocolate bar to the top and begin the melting process. The slower you melt, the shinier the chocolate. Stir every 5 seconds for uniform melting of the chocolate pieces. Upon reaching the melting point of 88°F, stop and allow cooling. It's not a bad idea to temper the chocolate at this point. If you're too impatient for tempering, you can use candy melt chocolate as a shortcut.
Coat the chocolate. When the chocolate is melted and smooth, use a spoon to portion it out into the chocolate mold. Then, using the same small spoon to make sure that the chocolate coats the entire mold cup. Alternatively, you can use a brush and brush the chocolate into the mold until it's coated. You want to have a decent 1/4" thick coating of chocolate in the mold. To get your chocolate bomb mold coated extra well, you can place the mold in the freezer and allow 2–3 minutes for the chocolate to set. Then, coat it some more!
Remove the chocolate spheres from the molds. Save half of your chocolate molds for Step 6. For the other 50%, remove the spheres from each chocolate mold. Stretch the edges of each silicon mold cup so that the chocolate pulls away from the silicone. Then turn the mold over onto a baking sheet or parchment paper and gently push on the top of the dome so that the chocolate half-spheres release from the mold. Next, put the half-spheres back into the mold so you can fill them. This will make it a lot easier when it's time to take them out of the chocolate bomb mold for good.
Fill the molds. For the 50% of chocolate spheres (half-spheres, technically) that you removed from the mold (and then gently replaced), you will now add fillings. Put 1 or 2 tablespoons of hot chocolate powder or hot cocoa powder in each chocolate sphere, followed by other fillings of your choice. You should be able to fit in about 5-6 mini marshmallows and some additional ingredients. Consider any of the following (or dream up additional options) for topping up your hot chocolate bomb mold: crushed cookies, mini chocolate chips, peppermint patties, powdered coffee, or anything else that might give an interesting flavor twist.
Assemble the two halves to make chocolate spheres. Carefully remove the remaining 50% of chocolate shells / half-spheres from the molds. Pipe some melted chocolate around the edge of the filled half-sphere, where the empty half-sphere will join, using your piping bag. Gently press the empty half-sphere onto the full one to create a complete chocolate sphere. Pop the assembled chocolate spheres back in the freezer to set.
Decorate the hot cocoa bombs. Decorating allows you to add a personal touch to each chocolate bomb! Melt the chocolate melts according to the package directions, then pull the bombs out of the freezer, pop them out of the molds (they should come out easily), and use the melted chocolate as a base to add sprinkles, glitter, or any other decorative touch you like! You can use the melted chocolate or opt for decorative balls and colorful sprinkles. Use gloves when decorating the chocolate bombs to prevent fingerprints. Wipe away excess chocolate or sprinkles, and pop in the freezer a final time to allow the decorative layer to set.