Add 1 cup of the white chocolate to a microwave safe bowl and heat for 30 seconds. Stir, then heat again for 10-15 seconds at a time, stirring in between, until melted and smooth.
Add about 2 Tbsp of the melted white chocolate to a half sphere mold cavity. Use a spoon or small pastry brush to "paint" the inside of the mold. Cover the interior of the sphere entirely. Repeat with remaining mold cavities and chocolate.
Place mold in the freezer for about 5 minutes, or until completely hardened.
Gently remove the half spheres from the molds and set on a work surface. Fill 3 of the half spheres with 1-2 Tbsp of hot cocoa mix, some chocolate chips, and some mini marshmallows.
Microwave a microwave safe plate for about 45 seconds and carefully remove (use an oven mitt if it gets really hot). Place an empty half sphere, open side down on the warm plate. It should only take a second or two for the rim to melt a little. Place that half sphere on top of one of the filled half spheres, then set aside to harden. Repeat with remaining cocoa bombs.
Once all the cocoa bombs are filled and assembled, melt the remaining 1/2 cup of white chocolate, heating for 15 seconds at a time, stirring in between, until melted and smooth.
Transfer melted white chocolate to a piping bag, or ziploc bag. Cut a small snip off the corner and drizzle the assembled cocoa bombs with the melted chocolate. While the drizzles are still melted, quickly sprinkle with desired sprinkles.
Set aside to harden, or add to the refrigerator to harden quicker.
To use: add one hot chocolate bomb to a mug and top with 8oz of hot milk. Stir until it has melted completely.