Place the dark 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 mint baking chips and 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 chocolate chips in 15 second intervals until melted, then place the remaining chocolate in a piping bag. Drizzle the chocolate over the top of the bombs and decorate with remaining mint pieces.
Allow the chocolate decorations to set before using, around 1 hour.