1. Place eggs and water to cover in a saucepan; bring to a boil. Remove from heat, and let stand, covered, 10 minutes. Drain eggs, and rinse with cold water for several minutes. Peel eggs, and cut in half lengthwise.
2. Place 6 egg halves (including whites) and all additional egg yolks in a food processor. Add mayonnaise, mustard, and 1/4 cup grated cheese; blend well. Add salt, pepper, and, if desired, truffle oil. If too thick, add olive oil (or more truffle oil if you want a stronger flavor), 1/2 teaspoon at a time, to desired consistency.
3. Place mixture in a pastry bag with a plain or star-shaped tip (or a small zip-top plastic bag with a corner cut out), and pipe mixture into remaining egg halves. Garnish, if desired.