Melt the milk chocolate by putting it in the microwave for 30 seconds. I use Callebaut. (Leave a bit to the side for sealing the chocolate).
Stir the chocolate until it melts well. (Be patient If it is not melted enough after a couple minutes of stirring, you can put it back in the microwave for no more than 15 seconds. That should be enough for it to fully melt after your stir it some more. Don't burn the chocolate!)
Use a spoon to paste the chocolate around the sides and bottom of the sphere molds. (Do not make it too thin, it will break).
Place the filled molds in the freezer for 5 minutes.
After 5 minutes, Take the molds out of the freezer and your chocolate should be hard.
Turn the mold upside down on a tray, and unfold the mold away from the chocolate by placing your thumb on the top center of the mold as you unfold.
Fill half of a chocolate ball with your chocolate powder, and marshmallows, if using.
Fill a decorating bag with melted milk chocolate. (You may have to reheat the chocolate a bit before putting it into the bag. You want the chocolate to be loose, not thick).
Use the decorating bag to pipe chocolate around the edge of the bottom half of the chocolate ball. (You can let some come out of the side so it's easier to help decorations stick to the sides).
Place another half of a chocolate ball on top of the filled half to seal.
Decorate the balls how you choose. (I rolled the sides in sprinkles to cover the seal lines. And used melted white chocolate and crushed peppermint to decorate the top).
Drop the chocolate balls in a cup of warm or hot milk, and watch it melt!