Place 8 oz. (or 2/3) of the chocolate pieces into the top of a double boiler or a glass/metal mixing bowl.
Heat an inch of water to a simmer in a small pot on the stovetop. Place the double boiler or bowl over the top of the simmering water (the top bowl should NOT touch the water). Stir with a spatula while the chocolate begins to melt.
Regulate the temperature of the chocolate carefully. Remove from heat as soon as the chocolate is melting, but there are still some solid pieces. Temperature should not exceed 105°F for white chocolate.
Continue stirring off of the heat until the chocolate is smooth. Add the remaining 1/3 of chocolate pieces, a little at a time, stirring until they are melted as well.
Continue stirring the chocolate until it cools to a temperature of 82°F.
Once the chocolate has cooled, place the bowl/pan back over the simmering water and reheat just a little bit until it is smooth and liquid enough for molding (85-88°F for white chocolate.) It is important to keep the chocolate between 82°F and 88/90°F at all times until it has been placed in the mold.
To make marbled hot chocolate bombs, follow the above steps using white chocolate. At this point, dab the tip of a toothpick or bamboo stick with green gel food coloring. Swirl the stick through the chocolate just enough to leave a trail of color behind. Do not overmix; it's important to keep separate green and white color swirls in the chocolate.
Spread a layer of melted chocolate to cover the full inside of each sphere cavity. Place the filled mold into the refrigerator for 3-4 minutes to set.
Gently remove each half sphere from the mold.
Heat a small plate in the microwave or cast iron pan. Prepare your workspace with the hot cocoa mix, Lucky Charm marshmallows, and gold sprinkles so you can work quickly.
Quickly melt the edges of one half sphere on the heated plate, fill with 1 tablespoon of hot cocoa mix, marshmallows, and sprinkles. Melt the edges of the other half of the sphere on the hot plate and seal the two sides together.
Drizzle the tops with melted chocolate and/or add sprinkles or decorations.
To serve: place one hot chocolate bomb in a mug or heat-proof cup. Pour 6-8 oz. of steaming milk over the hot chocolate bomb. Stir and enjoy!