Add water to a medium pot and place glass bowl on top of it. Wait for it to boil.
Add chopped dark chocolate to the glass bowl and turn heat to low. If the chocolate is melting quickly, set it aside on a potholder briefly. Stir chocolate gently with a spatula.
Keep the chocolate between 87°F and 91°F. This can be tricky so keep the thermometer handy and adjust accordingly.
Once you have the chocolate at the right temperature, remove from heat. Use your spatula or small brush to coat the half spheres with chocolate. Be sure not to miss a spot. Brush the chocolate just up over the lip of the opening.
Once the half spheres are coated well, put the mold in the freezer for at least 5 minutes. Put the melted chocolate back on the stovetop or warmer to keep at correct temperature.
Two coats work best, so remove molds from freezer and repeat the process adding a second layer of chocolate. This makes the shells stronger, which cuts down on cracking.
Take the mold out of the freezer again and gently push the chocolate out of the molds from the bottom/middle of each one.
You should now have 6 cocoa bomb halves. Fill 3 of them with 2TB of hot cocoa mix and 1 TB of mini marshmallows.
Spoon some melted chocolate onto a plate and press the rim of one half in the melted chocolate and rotate it gently (coating entire rim). Stick this half to a half with cocoa mix in it. Take your finger and gently smooth the chocolate around the seam. If you have small holes or cracks, dab melted chocolate into them.
Gently place the sealed cocoa bombs on parchment paper. A deviled egg dish is a great place to let them dry also, so they won't roll. Let them dry for at least 15 minutes.
Melt your white or dark chocolate to drizzle on them. You can use a decorating bag, fork or Ziplock bag to drizzle the icing on. Add your sugars or sprinkles quickly before the fresh chocolate dries.
Let them dry at least 30 minutes before moving them. More like 45 minutes if you are putting them into little bags.