Place the chocolate chips in a microwave safe bowl and heat at 30 second intervals. Stir chips between each interval, until the chocolate has melted completely.
Spread a spoonful of melted chocolate inside one cup on the silicone mold and use the back of the spoon to spread the chocolate up the sides of the mold. Continue this process until all of the molds are coated with chocolate. (Make sure the chocolate is nice and thick all the way up the mold, otherwise it will break or crumble when it's time to seal the bombs.)
Set the remaining chocolate aside for sealing and decorating the bombs.
Allow the chocolate in the molds to set and harden completely (approximately 3-4 hours). This process can be sped up by placing the mold in the refrigerator.
Once the chocolate has set, remove each chocolate piece from the mold and lay half of the pieces on your work surface to be filled.
Add some instant hot chocolate to half of the pieces (approx 1-2 TBSP to each half).
Top the mix with marshmallows.
Reheat the semi-sweet chocolate at 15 second intervals until melted.
Pick up an empty chocolate piece and dip the edges into the melted chocolate. Carefully place it on top of a filled piece and press lightly into place. Use your finger to smooth the melted chocolate around the seam. Continue until all of the chocolate bombs are sealed.
Place a small amount of melted chocolate on top of each bomb and place three marshmallows on the melted chocolate. Drizzle the remaining melted chocolate over the top of the marshmallows and sprinkle graham cracker crumbs on top.
Allow the hot chocolate bombs to set before using (about an hour).
TO ENJOY THE HOT CHOCOLATE BOMBS: Place a bomb in the bottom of a mug. Pour one cup of hot milk over the top of the bomb to melt the chocolate. Once the chocolate has melted completely, stir all of the mug's contents to combine. Enjoy!