Break the chocolate into small pieces and place it in a microwave-safe bowl. Microwave it for 20-30 second intervals, mixing at each interval. The chocolate can also be melted using a double-boiler.
Fill your mold ¾ of the way full with the chocolate and gently rotate it so that the chocolate coats all sides. Place a sheet of parchment paper onto your work surface. Turn the mold upside down and allow the excess chocolate to drip onto the parchment paper. Scrape the chocolate off the parchment paper and return it to your bowl.
Use the flat back of a butterknife to smooth the edges of the chocolate mold. Place the mold in the fridge for 20-30 minutes, until the chocolate has fully set.
Repeat steps 2 and 3, to create a second layer of chocolate. This will prevent the chocolate shell from breaking during unmolding and assembly. When smoothing the edges of the chocolate mold with the butter knife, ensure that the edges of the shell are 1-3 mm thick. This will allow both halves to securely stick together in the following steps.
Gently unmold the chocolate shells and rest them on a plate or on top of small glasses, which will support the spherical shape very well and keep it from wobbling away.
Make the hot chocolate mix by combining the cocoa powder and sugar in a small bowl. Spoon some of the mix into one chocolate shell. You don’t have to use all of the mixture. Add some marshmallows and sprinkles, if desired.
Re-melt some of the remaining melted chocolate and spread it onto the ridges of the filled shell. Place the other shell on top, press gently to seal and place the hot chocolate bomb into the fridge for 10 minutes, or until the chocolate seal has set.
To decorate, drizzle or spread some more chocolate on top. The chocolate should be melted, but cooled to room temperature, so that it doesn’t melt the chocolate shell. Sprinkle some sprinkles on top, if desired. Then chill the hot chocolate bomb in the fridge for another 10 minutes.
To serve, drop the hot chocolate bomb into a mug of hot milk. Stir and watch the marshmallow appear!