In a microwave safe bowl, heat 1 cup chocolate at 50% power starting for 30 seconds intervals for 1 minute (2 times) stirring between. Then melt at 15 second intervals until mostly melted. Do not overheat, if there are a few lumps, just keep stirring until smooth.
Measure about 2 teaspoons of chocolate into each half sphere and then brush it up the sides.
Place mold into freezer for 5-10 minutes to set.
On the second round, spoon 1 teaspoon of chocolate in each mold, brushing up the sides to create a sturdy wall. Work one at a time because chocolate will harden and seize before you get to all of them. Freeze again.
Equally divide hot chocolate mix and marshmallows into half of the spheres.
Pop out the empty halves.
Heat remaining 1/4 cup chocolate in the microwave in a candy piping bag. Snip edge to make a narrow opening.
Pipe along the edges and then place on top of filled halves to create a sealed ball.
While chocolate seams are still tacky, roll in sprinkles on a shallow plate.
After fully set and cooled, drizzle additional chocolate over top and decorate with more sprinkles or frosting.
When ready to serve, heat milk in microwave and place hot chocolate bomb into a mug.
Pour hot milk over bomb and stir until powder has dissolved.
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