Place white candy melts in a microwave safe bowl and melt in the microwave per the directions on the package.
Place 1 tbsp melted candy melt into one half of a bomb mold and, using the bottom of your tablespoon measuring spoon, press the chocolate up the sides of the mold, keeping it thick but even. Repeat with 12 bomb mold halves.
Place the chocolate filled bomb molds into the freezer for 5 minutes.
Remove molds from the freezer and gently tap the sides of the mold, pressing on one side of the chocolate to make the chocolate slide out of the mold.
Fill 6 of the molds with 1 tbsp hot chocolate mix, a few unicorn sprinkles, 1 tbsp sanding sugar in one color, and 1 rainbow marshmallow.
Heat a plate or a flat-bottom bowl in the microwave until the surface of the plate is warm to the touch.
Press one of the empty chocolate bomb halves open-side-down onto the flat portion of the warm plate for about 10 seconds. This will gently melt the rim of the chocolate cup.
Immediately press this warmed edge chocolate cup to the top of one of the filled cups. This will join the two halves of the hot cocoa bomb.
Using the pressurized icing, add a swirl of icing on top of the assembled hot cocoa bombs then immediately follow by adding a unicorn horn sprinkle on top and sprinkling additional sanding sugar over them.
Place the hot cocoa bombs into the freezer for 5 minutes to get them to set, then remove and store in an airtight container until you’re ready to use.