Chop chocolate chips as finely as possible, which will aid in more even melting.
Place ~5 ounces of chopped chocolate (about ⅔ of the chocolate) in a microwave safe bowl. Microwave on the defrost setting in 15 second increments, stirring between each. When most of the chocolate is melted, with a few pieces still solid, stop microwaving and stir to finish melting the chocolate. The temperature for semi-sweet chocolate should be between 100-113°F at this point and not hotter than 120°F.
Add the remaining chocolate pieces and stir until they are mostly melted and incorporated. Continue stirring the chocolate until it cools to about 85°F.
Place the chocolate back into the microwave on the defrost setting for 15 seconds once more. This should be enough time to heat the chocolate back up to 90° and make it fluid for molding.
Use a paintbrush or the back of a spoon (a soup spoon works great!) to coat the inside of each sphere of the candy mold. Apply two thin coats or one thicker coat, between 1/16” and ⅛”.
Use a knife or straight edge to clean up any excess chocolate on the surface of the mold.
Place the mold in the refrigerator for 3-5 minutes until the chocolate sets.
Gently pop the molded chocolate from the silicone mold.
Spoon 1 ½” teaspoons of hot cocoa mix powder into one side of each sphere half. Add as many mini marshmallows as you can.
Heat an empty plate in the microwave for about a minute and place it next to your work space.
Place the empty half of each sphere, open side down onto the plate for just a couple seconds to melt the rim. Quickly press the melted rim against the rim of the other half to seal the sphere together.
You can alternately paint a bit of melted chocolate on the edge to glue the two sides together, but this leaves more of a visible line.
Place white chocolate chips in a piping bag or Ziploc bag. Microwave in 20 second increments, squishing the chocolate within the bag, until it is fully melted. Snip off a small corner of the bag (or the tip if using a piping bag).
Drizzle the white chocolate over the top of each hot chocolate sphere for decoration.