Spread ½ of the salt in a layer in a container such as a tupper-ware, the container doesn't matter that much, just so long as the yolks don't get sloshed around, since they would break. I have used cookie sheets, tupper-ware, and ½ pint canning jars, they all worked fine, but the ½ pint canning jars were probably the easiest to use.
Crack the duck eggs one at a time in your hand, allowing the white to fall through into another dish. Save the whites for an omelet or something, or discard.
Gently place the yolks on top of the salt and then cover with the rest of the salt, making sure they are completely covered. Refrigerate the yolks for 2 days.
After two days, remove the yolks from the salt and wrap in cheese cloth. Tie up the yolks and hang them to dry for 1 week in a cool dark place. After a week, remove the yolks from the cheesecloth, wrap in plastic and refrigerate. To serve, grate the yolks as you would cheese (preferrably with a microplane grater as they are salty).
Notes
Duck eggs or goose eggs are the best for this since they are so large, but chicken eggs make a fine substitute. If you don't have fresh truffles, don't worry. Truffle salt can be purchased at many specialty markets, I even see it in the discount aisle at Marshalls and T.J. Maxx.Like I mentioned, you don't have to use truffles either for this, making perfumed salt blends with herbs will give great results too.A little bit of cured yolk goes along way, so I'm scaling this to two yolks, which will be more than enough to garnish 10 plates or more, depending on what you're doing.