Melt the chocolate by putting in a microwave proof bowl and cooking for 30 seconds, then stirring for about 30 seconds. Repeat (probably twice) until all is smooth and runny.
Coat the inside of your sphere moulds. Turn them over and tap the base to shuffle the chocolate down the sides.
Turn back over, check for any gaps that have appeared (fill with more chocolate if there are any) and pop into the freezer for about 15 minutes.
Put gloves on if you're using. Remove the half spheres from the freezer and pop out of the moulds.
Fill half of them with the hot chocolate powder and marshmallows. (Don't overfill or they will be harder to close.)
Warm a dinner plate (either in a hot oven for 2 - 3 minutes or by running under a very hot tap for a couple of minutes and drying off - don't let water touch chocolate!)
Put the edge of one of the unfilled shells onto the warm plate very briefly. Use a knife to lift it off and sit it on top of one of the filled shells.
Push together in your hands and smooth the edges with your fingers.
Put back into the freezer for 15 minutes.
Melt your additional chocolate and drizzle all over the bombs. Add sprinkles if you like and chill until set.
Heat up a mug of milk until very hot. Drop in a bomb and watch it do it's stuff! Stir well before drinking.
Note 1 - Chocolate
These measurements are for the larger, 70mm moulds I mentioned. You will need to adjust depending on which mould you are using (and if you want to do 2 coats of chocolate or not).
Note 2 - Hot Chocolate Powder
Be sure this is drinking chocolate powder, not cocoa (which has no sugar added).
Note 3 - Chocolate to decorate
You can mix this up and use white or dark chocolate if you prefer.
Note 4 - Oil
You only need to add this if your drizzling chocolate feels to thick to give a nice stripy drizzle. It just thins it down.
Note 5 - Sprinkles
Use colourful sprinkles or chocolate ones. Go wild!