1. To make aioli, place egg yolk in a bowl (do not use stainless steel). Slowly add oil in a thin stream, whisking constantly until thickened. Add vinegar and truffle or truffle oil.
2. Preheat grill to medium. Bring a pot of salted water to a boil, and prepare a bowl of ice water.
3. Snap off tough ends of asparagus spears. Wrap a strip of pancetta around middle of a pair of spears. Grill wrapped asparagus bundles for 6 minutes or until pancetta is crispy.
4. Using a 2 1/2-inch biscuit cutter, remove middle of each bread slice; discard edges. Brush both sides of each round with butter. Grill bread on each side; set aside.
5. Cook eggs for 6 minutes in boiling water; place in ice bath to cool. Peel eggs and cut them in half, being very careful not to let yolks fall out.
6. Place 2 bread rounds on each of 4 serving plates; top with some truffle aioli. Place 1 bunch asparagus on each bread round; top with an egg half.