Why does the chocolate coating on a Klondike bar melt to liquid at room temperature, but the chocolate of a candy bar remains solid?
The chocolate you eat has a lot of fat in it. Saturated fat has a higher melting point than unsaturated. A candy bar has more saturated fat in it than the coating on a Klondike bar, so it stays solid at higher temperatures. It’s kind of like the difference between olive oil and Crisco, both of which are pure fat.
Actually, it has more to do with the percentage of cocoa butter. Higher cocoa butter content generally equals a lower melting point. Mass produced chocolate typically uses other fats in place of some of the cocoa butter because they are a) cheaper, and because they want a higher melting point to improve shelf life and reduce spoilage during shipping.
In addition to composition, the melting point of chocolate is also influenced by how it has been tempered.
The very finest quality chocolate has a melting point slightly below body temperature. This makes handling and storage more difficult, which is one reason you pay big bucks for gourmet chocolate. (In addition to being designed to handle greater extremes of temperature, Mars bars and the like are intended to have a long shelf life whereas gourmet chocolate typically has no preservatives and must be consumed within a week or to of production even if it has been refrigerated.)
It is, however, worth it. Properly prepared chocolate begins to melt as soon as it touches your tongue and releases a complex mix of volatile compounds that create a sort of cooling sensation. Chocolate like this is to mass-produced chocolate what the finest espresso is to instant coffee.
N.B. Proper gourmet chocolate “melts in your mouth.” This is not to be confused with the “melting” that occurs due to the enzymes in your saliva disolving the sugar in the chocolate. It’s hard to adequately describe the difference, but once you’ve experienced the real thing, you’ll know. 