Break the 8 oz chocolate bars into small, even pieces.
Place the chocolate in a microwave-safe bowl.
Microwave the chocolate at 50% power in 30-second increments, stirring after each increment until almost completely melted.
Remove from the microwave and stir gently to finish melting the chocolate.
Choose 12 half sphere molds and ensure they are clean and dry.
Use a small spoon or pastry brush to coat the inside of the molds with the melted chocolate. There will be some remaining melted chocolate, just set aside for now.
Refrigerate the coated molds for about 5-10 minutes until the chocolate sets.
Carefully remove the molds from the refrigerator.
Fill half of the molds half with 1 tablespoons of hot cocoa mix and mini marshmallows.
Remelt the remaining chocolate and place in a piping bag or in a zip top bag.
Snip a tiny hole at the tip of the piping bag or corner of the zip top bag. Pipe a ring of melted chocolate onto then half sphere that is filled with hot cocoa mix and marshmallows.
Immediately press matching halves together to seal the cocoa bomb.
Drizzle any remaining melted chocolate over the top of the cocoa bombs for decoration.
Allow the cocoa bombs to set and harden for about 10-15 minutes at room temperature or in the refrigerator.
To use, place one cocoa bomb in a mug and pour hot milk over it. Watch it dissolve and release the cocoa mix and marshmallows.
Stir and enjoy your delicious hot cocoa!