Place white candy melts in a microwave safe bowl and melt in the microwave per the directions on the package. You can also melt chocolate in a double boiler.
Place 2 tablespoons of melted white 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 for a total of 6 bomb mold halves.
Place red candy melts in a microwave safe bowl and melt in the microwave per the directions on the package.
Place 2 tablespoons melted red 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 for the remaining 6 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 slid out of the mold.
Fill the red molds with 2 tablespoons hot chocolate mix and 6-8 mini marshmallows.
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 white 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 red cups. This will join the two halves of the hot cocoa bomb.
Using a piping bag or a fork, pipe a big, fluffy line of white candy melts over the join line of the assembled hot cocoa bombs as well as a dollop on top.
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.