this is accomplished with a product referred to as "candy glass". you can buy it in sheets, bottles, mirrors and all sorts of things. What you do NOT want to do is try to make it yourself. while it is possible to melt sugar down or start with a corn syrup mixture and cast it, I can't recommend trying it. You will almost certainly end up with either a lumpy yellowish mess that won't work or just burnt sugar. If you do manage to get it in a mold, it will also likely shatter or crack while it cools. I know it can break the budget of an amature film, but it also lends production value to use the real deal and invest in some breakaway glass that is not only visually exciting, it is SAFE. here's a pretty cool company that manufactures all sorts of breakaway products http://www.breakawayfx.com/