How can I prevent chocolate from hardening when I add it to cream?
When you add warm melted chocolate to anything cold, it instantly hardens–usually in the shattered looking form you describe. To prevent this from happening, you can melt the chocolate with the liquid (be sure there is a minimum of 1 tablespoon of liquid for every two ounces of chocolate or the chocolate will seize. Or you can warm the liquid before combining the two.
To add melted chocolate to an ingredient like whipped cream or ice cream that can't be warmed, you will need to make the chocolate into a stable sauce by adding butter, cream or other liquids to it first.