Cooked chocolate icing is warm and thin immediately after coming off the heat, but it cools quickly to a thick consistency. In most cases, the trick is to spread it after it's cooled enough that you can control it, but before it hardens completely. Consider the type of cake and your purpose, though, because some icings should be spread immediately.
In the Box
For casual sheet cakes served in the pan, you can pour the warm chocolate icing directly on a warm chocolate cake. This is the classic method for icing a Texas sheet cake. The icing glides over the cake, but stays put because of the barrier made by the pan. Allow the cake to sit at room temperature for at least 2 or 3 hours, until the cake has completely cooled and the icing has set.
If you're icing a round cake, you'll probably want to cool the frosting to room temperature before applying it to the cake. To use a regular cooked chocolate icing, allow it to cool to room temperature for 1 hour or so, until it reaches the consistency of peanut butter. Then spread it carefully over the cake. Allow ganache -- which is made of nothing but heavy cream and chopped chocolate -- to cool covered for at least 8 to 10 hours. At this point, it thickens and spreads beautifully. Use it as a thick filling and an icing.
Allowing ganache to cool completely makes a thick, luxurious icing. Sometimes, though, you want a thin, glistening glaze. Ganache fills this role, as well. To make a glaze, allow the ganache to cool for 15 to 20 minutes -- until it's the consistency of warm honey. Pour the ganache over the cake and smooth it gently. The ganache runs down the sides of the cake. To simplify clean-up, place the cake on a cooling rack set over a baking sheet before you pour the glaze. The baking sheet catches the excess ganache. Allow the glaze to harden before you transfer the cake to a cake platter. Scrape the remaining ganache off the baking sheet and save it for another purpose.
Cool a cooked chocolate icing or ganache to room temperature before spreading it atop cheesecake or other baked goods that contain soft or perishable ingredients. If you pour hot ganache or frosting over these cakes, they're likely to melt. Keep these desserts refrigerated at 40 degrees Fahrenheit or cooler.