Place the chocolate into a medium bowl. Heat the cream in a small sauce pan over medium heat. Bring just to a boil, watching very carefully because if it boils for a few seconds, it will boil out of the pot. When the cream has come to a boil, pour over the chopped chocolate, and whisk until smooth.
Tap the bowl on the counter to settle the chocolate into the cream, then let it sit for 1 minute. Using a rubber spatula, slowly stir in a circular motion, starting from the center of the bowl, and working out to the sides. Be careful not to add too much air to the ganache. Stir until all the chocolate is melted, about 2 minutes. It may look done after 1 minute of stirring, but keep going to be sure it's emulsified.
Let the ganache sit at room temperature until it cools to 70 degrees F. In a 65 degree F room, this will take approximately 4 hours or 2 hours in the refrigerator. You can speed up the process by pouring the ganache out onto a clean baking sheet (thinner layers cool faster.) Once the ganache reaches 70 degrees F, it is ready to be used. At this point it can be covered and stored in the refrigerator for up to 2 weeks.
Notes
Notes:
To get the "mirror cake" effect, pour over the cake while still slightly warm, starting at the center of the cake and working outward.
For a fluffy frosting or chocolate filling, allow it to cool until very thick, then transfer to the bowl of a stand mixer fitted with a whisk attachment and whisk until light and fluffy.