Using a ladle, fill 2 cork molds with chocolate. When they are full, empty the excess back into the bowl of chocolate. The inside of the molds should be evenly coated with chocolate. Wipe the lip of the molds clean and place them upside down on a wire rack over a baking sheet to drain. Once the chocolate starts to harden, about 5 minutes, use a wide pastry scraper to clean the edges of each cavity clean. This is important because when the chocolate sets, it shrinks or retracts from the sides of the mold. A clean edge will keep it from sticking and cracking as it shrinks. You can place the mold in the refrigerator for several minutes to help the chocolate to harden.
Place the champagne and cream into a small saucepan and heat the mixture to a boil. Pour the hot champagne mixture over the chopped chocolate and corn syrup and blend until smooth with an immersion hand blender. Add the marc du champagne and the butter and blend until smooth with an immersion hand blender. Allow the mixture to cool until it has a thick consistency (thick enough to pipe). Place the mixture in a piping bag and pipe it into the chocolate filled molds. Let set overnight. Close the bottom of one mold by apply chocolate with an offset spatula. Scrape clean then press the two molds together evenly. Allow to set. When the chocolate has set, remove the whole "corks" from the molds.
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.