Add truffle to a tamago sando and you are making a deliberate argument: that the cheapest, most democratic sandwich in Japan can be dressed up into a luxury object without losing what it is. The base is still mashed egg bound in kewpie mayonnaise between soft crustless shokupan, but it is perfumed with truffle, either truffle oil worked through the filling or shaved fresh truffle laid over the egg, and the price tag moves accordingly. The effect, when it works, is the familiar cool savory creaminess of the egg lifted by a deep mushroomy musk that hangs at the back of every bite. It is the same sandwich wearing a different coat, and the whole point is the tension between humble form and extravagant scent.
The craft is restraint, because truffle is the easiest ingredient in this section to ruin. The egg is prepared the usual way, hard-cooked and chopped or pressed, bound with enough kewpie that it stays glossy and cohesive, and then truffle is introduced with a light hand: a measured amount of good truffle oil folded in, or thin shavings of fresh truffle arranged so they are visible and aromatic without overwhelming the mild egg underneath. The shokupan stays trimmed and untoasted so nothing competes with the perfume. A good one smells of truffle before you taste it and then delivers egg first and musk second, the two in balance, the filling spread wall to wall so no bite is bare. A sloppy one is dominated by synthetic truffle oil that tastes of petrol and lingers unpleasantly, or by so little truffle that the upcharge buys nothing, or by an egg base so under-seasoned that the truffle has nothing to sit against and the whole thing reads as expensive and empty. The bind is the same discipline as the plain version, the filling holding the slices together and staying creamy without weeping, but with an added burden: the aromatics have to be evenly distributed so the truffle is not concentrated in one corner and absent from the rest.
That puts this firmly in the premium-treatment branch of the tamago family rather than the regional one, a sandwich about indulgence rather than technique of the egg itself. The variations move along the source and form of the truffle: black truffle for a deeper, earthier register, white truffle for something sharper and more garlicky, truffle paste blended through for an even musk versus fresh shavings for aromatic punctuation, and some builds that pair it with a richer soft-scrambled egg rather than the firm mash. Each of those changes the luxury calculus in its own way and deserves its own article rather than being crowded in here.