Chocolate's temperamental nature in the kitchen arises because of its structure. When you cook with sweetened or semisweet chocolate, you're dealing with a complex blend of fats, sugars and cocoa solids that take on different characteristics depending on their environment. Combining chocolate with other ingredients in an icing adds even more variables to the equation. When chocolate icing fails to harden, the usual culprits are variations in ambient conditions.
Humidity
Chocolate contains relatively little water, but the sugar in chocolate and chocolate-based foods has a hygroscopic effect -- that is, it draws moisture from the air around it. When the humidity rises above 50 percent, more water becomes available to be drawn to the sugar and therefore into the chocolate mixture. Icing that starts with solid chocolate requires low humidity to set fully; if the weather's too humid, the excess moisture will find its way into the chocolate and keep it from setting.
Ambient Temperature
The cocoa butter in chocolate melts at a temperature just below your body temperature; that's why chocolate melts in your mouth so readily. Although it melts at roughly 93 degrees F, cocoa butter turns soft at lower temperatures. If your icing recipe contains whole chocolate with its included cocoa butter, it will soften in a hot kitchen and refuse to set. Combining chocolate with dairy butter or cream in a frosting or ganache complicates the melting issue still more. Keep chocolate icing cool to maximize your chances of proper hardening.
Surface Temperature
Even on a cool day, icing a cake or cookies that are still warm from the oven will keep chocolate-based icings from setting. While the surface of the cake may feel cool to an initial touch, if the heat radiating from the center of the food approaches that 93-degree mark, chocolate coatings will melt. If you can't cool your cake or cookies completely before icing, then refrigerate the partially cooled sweets for at least half an hour to cool the surface thoroughly.
Fat Content
An icing with too much fat -- whether from cocoa butter, dairy butter or heavy cream -- becomes more prone to improper setting. A high butter percentage in your icing prevents hardening and keeps a frosting pliable. If you want a finished product that remains spreadable and soft, that's a good thing; if you want icing to harden to a firmer texture, it could be a drawback. For firm icing, you need a recipe that uses less fat. Lowering the fat content in icing also improves its nutritional profile, although large quantities of sugar still make it an occasional treat and not an everyday food.
Corn Syrup, Honey and Other Liquid Sweeteners
Chocolate icing hardens as the cocoa butter in it turns firm and tiny sugar crystals form. Liquid sweeteners such as corn syrup, agave nectar and honey help delay the crystallization process in sugary foods. While this is beneficial if you're making fudge or taffy and want the product to remain pliable in its uncrystallized state, an excess of these liquid sweeteners can prevent your icing from hardening. If your recipe calls for sugar, use sugar instead of trying to replace it with a liquid sweetener.
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