Fill a pot with water, about 1/4 full. Use a metal bowl, place it inside the pot, and pour in half a bag of milk chocolate chips. Also, add a few drops of cooking oil. This will help temper your chocolate.
Heat over medium-low heat, do not stir. Make sure the hot water in the pot does not touch the bottom of the metal bowl.
Once they look soft, remove from heat and gently stir until you have a creamy consistency.
Spoon equal amounts into the mold cavities. Not too thick and not too thin. You want your hot chocolate bombs to easily melt under the hot milk, but you don’t want them to be so thin they will collapse as you fill and fuse the sides together. This part takes a bit of trial and error.
Use the back of a spoon to coat the cavity. Make sure you get all the way up to the top – and that this area is not too thin.
Pop them in the fridge or freezer. After they are cooled and solid, carefully remove them from the molds. Break off any stray pieces of chocolate. Try to have smooth edges around the top.
Fill half of your chocolate halves with Mexican hot chocolate, marshmallows, and whatever else you want to be in the final drink.
Using a mildly warm pan or plate, take one empty half and place it on the warmed plate or pan and then quickly connect it to a filled half. Quickly, use your finger to seal the joint with the melted chocolate.
Drizzle melted chocolate on top and add sprinkles. Put them back in the fridge to harden. After that, they are ready to serve!