Microwave two-thirds of the melting wafers at 50% power in 30-second bursts, stirring between each, until smooth.
Spoon about 1 tablespoon of chocolate into each mold cavity and spread it up the sides to the rim. Freeze 5 minutes.
Add a second coat, focusing on the rims. Freeze 5 to 10 minutes until firm.
Gently peel the shells out of the mold and set them on parchment paper.
Fill 6 halves with 2 tablespoons hot cocoa mix and a few mini marshmallows.
Warm a plate for 1 minute. Touch the rim of an empty half to the plate for 2 to 3 seconds, then press it onto a filled half to seal.
Drizzle with melted contrasting chocolate and add toppings. Let set at room temperature.
To serve, place a bomb in a mug and pour 8 to 10 oz of hot milk over it. Stir and enjoy.
Notes
Use the remaining third of the wafers to rewarm chocolate for the second coat and to patch any seams.Store in an airtight container at cool room temperature for up to 2 to 3 weeks.