You're going to want to begin by chopping up the Twix into small bite-sized pieces.
Then add the chocolate candy melts into a microwave-safe bowl and melt per the package directions.
Once melted, take and put 1 tablespoon of the chocolate into the molds. Make certain to spread it around and up the sides. Do this for all 12 of the molds.
After all the molds are filled, take and put all of them in the freezer for 5 minutes.
After 5 minutes of time, go ahead and remove them out of the freezer and give the sides a gentle tap. This should loosen them to remove them from the molds.
Fill half the molds with 1 tablespoon of hot chocolate mix, 1 tablespoon of the chopped candy bar, and 8 mini marshmallows.
Put a microwave-safe plate in the microwave and heat the plate. You want to heat it until it's warm to the touch. Once done, take and grab an empty mold and put it on the plate to melt the open-faced edge. You can then place it on top of the other half of the filled mold to seal.You can also use melted chocolate on the edges to connect.
Drizzle melted chocolate over the top of the coco bombs and then add on a piece of Twix on the top. Pop in the freezer for 5 minutes to firm up.