Step 8: Ganache Layer: Mix it up!
The key to tempering chocolate is to proceed with caution. I like doing this in the microwave, because it is easy to finely control the amount of heating the batch of chocolate gets.
The method used here is known as "incomplete melting." The goal is to carefully melt out the unstable cocoa butter crystals, seed the remaining batch with a small amount of reserved chocolate.
Weigh out the 396 grams of chocolate. Remove about 20% of it (~80 grams), chop it into smaller pieces with a knife, and set it aside.
Use short (10-20 second) bursts of microwaves to heat the chocolate. Between each period of irradiation, stir the chocolate around to make sure there are no hot spots forming. Do not let the temperature of the chocolate rise above 97 °F (36 °C).
If the temperature of the chocolate rises above 97 °F, it will lose its temper. Angry chocolate is bad chocolate.
Once the main batch is uniformly melted, mix in the chopped "seed" chocolate. It should melt in, seed the batch with the proper form of cocoa butter crystals, and bring the whole lot down to the proper working temperature. You can test the chocolate by smearing a bit on a piece of aluminum foil - it should set quickly, and without white streaks.
Massage the butter into the chocolate, until no lumps remain. Pour the 105 °F cream over the mixture (heat it up if it's cooled too much), and stir in a figure-eight motion until you get a smooth, uniform ganache.
Pour the ganache into the frame and smooth the surface. Allow it to cool a bit, cover with parchment paper, and leave to crystallize overnight.