Place a large plate in the freezer - this will help to keep the candy shells stay cool when assembling.
In separate bowls, melt the black, purple, green, and orange candy melts as directed on the package.
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Randomly drop a little of each color melted candy into one cavity of a medium/large (2.5") hot cocoa bomb/dome mold. Lightly swirl the back of a spoon while coating the mold. Do this until all 6 cavities in the molds are covered. Make sure you cannot see the mold through the candy melts. Place in the freezer for 10 minutes.
Take the molds out of the freezer and coat only where you can see through the mold. If you cannot see through the mold you do not need to add more. You want the bomb to stay together but not be too thick. If you add more melted candy, place the molds back in the freezer for another 10 minutes.
Remove the plate from the freezer.
Remove and carefully pop out each half of the bombs and place them on the chilled plate.
There are three methods for melting the rim of each sphere to even them out and for attaching the top half-shell to the bottom one:
Method 1: Pour some melted chocolate into a disposable piping bag, then pipe it around the edge of the bottom shell. Carefully place the top shell onto the bottom one and gently hold them in place for 30-60 seconds to allow the piped chocolate to harden and "glue" the pieces together.
Method 2: Warm up a shallow frying pan on the stove. Place a piece of wax paper into the pan, and holding the top half-shell, rub gently on the wax paper until the edges start to melt a bit. Use this to then stick the top to the bottom, holding together until set
Method 3: Heat a plate in the microwave for approximately 60 seconds. Remove it from the microwave then place it directly onto the hot plate for 5 to 10 seconds. You just want the rim to gently melt. (My preferred method.)
Before filling the hot cocoa bombs, use one of the methods above to gently melt the edges of each dome to even them out.
Spoon a serving of hot chocolate mix and a few mini marshmallows into 3 half-spheres.
Gently melt the top half as above and place one on top of each filled half. This will seal them together evenly. Use your finger to gently rub any melted candy to fill in any gaps. You want a firm seal without holes.
Attach a candy eyeball to each hot cocoa bomb using a small blob of almost cooled melted candy. Do not use hot melted candy as it will melt the bomb.
Serve by placing a Hocus Pocus hot cocoa bomb in a mug and pouring hot milk over to melt. Stir well. Enjoy and Happy Halloween!