Place the semi-sweet chocolate chips in a microwave safe bowl and microwave in 30 second intervals, stirring between each interval, until the chocolate chips are fully melted.
Put 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 the molds are coated.
Set the remaining chocolate aside for sealing and decorating the bombs.
Allow the chocolate to set completely, around 2 to 3 hours. You can speed up the process by placing the chocolate in the refrigerator.
When the chocolate is set, remove each of the chocolate pieces from the mold. Add a small amount of instant hot cocoa mix to half the pieces and top the hot cocoa mix with marshmallows.
Reheat the chocolate in 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 lightly press it into place. Use your finger to smooth melted chocolate around the seam. Continue until all the chocolate bombs are sealed.
Melt the white chocolate chips in 15 second intervals until melted, then place the remaining semi-sweet chocolate and white chocolate in piping bags. Drizzle the chocolate over the top of the bombs to decorate.
Allow the chocolate decorations to set before using, around 1 hour.
To make the hot chocolate bombs, place the bomb in the bottom of a mug. Pour one cup of hot milk over the top of the bomb to melt the chocolate. When the chocolate is completely melted, stir to combine.