Preheat the oven to 350 degrees F.
Place the nuts in a large mixing bowl. Add the corn syrup and stir until evenly coated. Place the nuts on a baking sheet and roast in the oven for 15 to 20 minutes. Remove from the oven. Using a wooden spoon, transfer the nuts onto a parchment paper-covered baking sheet pan. Do not touch them, as they are extremely hot. Let them cool at room temperature, or if possible, place them in the freezer because the cold nuts will help the chocolate to temper. When completely cooled, break apart any nut clusters that may have formed. Place the cooled nuts in the coating pan or in a large mixing bowl. Slowly add one third of the bittersweet chocolate, one ladleful at a time. If you are using the mixing bowl, fold the nuts until they are thoroughly coated and the chocolate has set. If you do not fold immediately, the chocolate will set and the nuts will stick together. Add another third of the chocolate and fold thoroughly until set. Add the remaining third and fold thoroughly being sure all the nuts are well coated. Separate any clusters of nuts that may have formed. If you serve the nuts as they are, let the chocolate set completely. If you decide to move on to the next step, do not wait for the chocolate to set completely. Add the powdered sugar or cocoa powder and stir until all of the nuts are well coated. If you'd like to coat half of the nuts in powdered sugar and the other half in cocoa powder, start with the powdered sugar. Before serving, place the nuts in a sieve to remove any excess sugar or cocoa powder. The nuts will keep at room temperature in an airtight container for up to 2 weeks.
How to Temper Chocolate(From Dessert Circus, Extraordinary Desserts You Can Make At Home by Jacques Torres):
Chocolate is tempered so that after it has been melted, it retains its gloss and hardens again without becoming chalky and white (that happens when the molecules of fat separate and form on top of the chocolate). There are a variety of ways to temper.
One of the easiest ways to temper chocolate is to chop it into small pieces and then place it in the microwave for 30 seconds at a time on high power until most of the chocolate is melted. Be very careful not to overheat it. (The temperature of dark chocolate should be between 88 and 90 degrees F, slightly warmer than your bottom lip. It will retain its shape even when mostly melted. White and milk chocolates melt at a temperature approximately 2 degrees F less because of the amount of lactose they contain.) Any remaining lumps will melt in the chocolate's residual heat. Use an immersion blender or whisk to break up the lumps. Usually, chocolate begins to set, or crystallize, along the side of the bowl. As it sets, mix those crystals into the melted chocolate to temper it. A glass bowl retains heat well and keeps the chocolate tempered longer.
Another way to temper chocolate is called seeding. In this method, add small pieces of unmelted chocolate to melted chocolate. The amount of unmelted chocolate to be added depends on the temperature of the melted chocolate, but is usually 1/4 of the total amount. It is easiest to use an immersion blender for this, or a whisk.
The classic way to temper chocolate is called tabliering. Two thirds of the melted chocolate is poured onto a marble or another cold work surface. The chocolate is spread out and worked with a spatula until its temperature is approximately 81 degrees F. At this stage, it is thick and begins to set. This tempered chocolate is then added to the remaining non-tempered chocolate and mixed thoroughly until the mass has a completely uniform temperature. If the temperature is still too high, part of the chocolate is worked further on the cold surface until the correct temperature is reached. This is a lot of work, requires a lot of room, and makes a big mess.
A simple method of checking tempering, is to apply a small quantity of chocolate to a piece of paper or to the point of a knife. If the chocolate has been correctly tempered, it will harden evenly and show a good gloss within a few minutes.