Melt the chocolate in a glass bowl in the microwave. Use 15 second bursts, stirring in between until everything is smooth and pourable. It will take about 1-2 minutes.
Scoop some chocolate into the mold and use the back of a spoon or pastry brush to push the melted chocolate around the molds, making it thick enough along the sides and edges.
Place the molds into the freezer for 5-10 minutes or in the fridge for 30 minutes to set. A couple of minutes into them setting, take them out and brush/spoon extra chocolate on the top edges to make it thicker. Let set completely, then carefully pop the chocolate dome out of the mold and set aside on a cold plate.
Microwave an empty plate for 30 seconds to 1 minute, until warm, but not hot. Take one chocolate dome and place it on the plate for a couple of seconds to melt the edges. Working quickly, flip it around and add 1 tablespoon hot cocoa powder and mini marshmallows and any other add-ins you want.
Take another dome and melt its edge on the warm plate. Join the two domes together into a sphere and hold until sealed. Let set in the fridge or freezer while you make the rest of your spheres.
To serve: Put into a mug, pour on warm milk (or hot chocolate!) and watch the magic! Stir everything up and enjoy.