This problem is not related temperature. It's related to physics, and understanding the behaviour of the chocolate you are working with.
When making chocolate confections, you have two compounds with different properties and different behaviours: You have your centers, and you have your coatings.
As the centers warm and cool, they just sit there and for the most part, do nothing. However it's important to keep in mind that some centers are softer than others, and also have more air incorporated into them than others, while other centers are dense, and less inclined to give in to slight compression.
Then you have your chocolate. THIS IS THE CULPRIT. As the cocoa butter in the chocolate crystalizes, it shrinks (which is why it comes out of molds very easily when properly tempered). Chocolate with a high cocoa butter content (such as good quality couverture) shrinks LOTS, and when properly tempered, has zero maleability.
Your truffles are rolled round.
When a sphere shrinks, it shrinks inward.
You cover a round ball with tempered chocolate, and as the chocolate sets it tries to shrink. However when it can't, something has to give, and as a result a crack appears. It really IS that simple.
A very soft center with air incorporated into it (such as a whipped/piped ganache), will allow the chocolate to compress it and as a result will not crack. A hard center will not give the chocolate an opportunity to shrink, and as a result many will crack.
Understanding the behaviour of your ingredients, you have a number of options:
- Use a couverture that has less cocoa butter
- Incorporate air into your centers by whipping them
- Double dip your confections (the first dip will crack, but the second generally won't)
- coat your truffles with something (which adds structure for the chocolate to grab on to and compress)
- Use a milk chocolate to coat your confections
This lesson here is further supported by the process of molding chocolates. The molds are poured, emptied, and scraped, and allowed to set before the filling is piped in (this gives the thin chocolate shell time to shrink). The center is piped in, and the bottoms are poured. Thick bottoms can cause cracking whereas uniform bottoms very seldom ever do.
Cheers and Happy Chocolate Making
Brad