Set up a double boiler with a heat-safe glass bowl that fits over a small pot on the stove. The glass bowl shouldn’t touch the water.
Heat about an inch of water in the pot over medium heat. Place the chocolate wafers into a heat-safe glass bowl and place it on top of the pot. Stir the chocolate with a silicone spatula as the chocolate melts. Do not allow the chocolate to get too hot or it will scorch. Remove from heat as soon as chocolate is half melted. Check the temperature of the chocolate with a digital thermometer. Don’t heat the chocolate to more than 120°F. If you remove it when half melted, it should reach about 105°F as you stir. Continue stirring until all the chocolate melts.
Once the chocolate has cooled to 82°F (about 5-10 minutes), place the bowl back on the double boiler and bring it back up to 87°F. Maintain this temperature as you work, if possible. Use the back of a spoon or a pastry paint brush to coat the inside of each well (12 total). Place the molds in the refrigerator for 5 minutes. Give a second coat of chocolate to any spots that look thin or have holes. Refrigerate for 5 more minutes.
Remove the chocolate shells from the molds.
In a small bowl, mix together the Bailey’s Irish Cream and hot chocolate mix.
Heat a small pan on the stove then turn it off. Quickly melt the rim of 6 of the spheres. Place them, cup facing up, on a flat surface. Fill each with 1 tablespoon of Bailey’s mixture and 1 tablespoon mini marshmallows. Now quickly melt the rim of the other 6 spheres and place them onto the filled half to adhere them together.
To decorate, load the remaining tempered chocolate into the corner of a small plastic sandwich bag. Cut off a tiny corner. Drizzle melted chocolate over the tops of your spheres and decorate with sprinkles. If desired, lighten the color of the drizzle by adding a few white chocolate wafers to the chocolate and melting them in.
To serve: Steam or microwave 8 ounces of milk. Place one hot chocolate bomb in a mug of steamed milk. Wait until the chocolate melts and the bomb bursts. Stir and enjoy.