This month Burger King in Hong Kong released the Black Truffle Angus XT as part of their premium burger line, the King's Collection. The burger is topped with mayonnaise mixed with Italian black truffles, along with cheese, bacon, tomato, and curly endive. From Burger King's ad:
King's Collection Exquisite Indulgence: The exquisite aroma of the black truffle, otherwise known as "black diamond" forms a delectable combination with our extra thick and juicy Angus beef.
The burger costs HK$38 (about $4.90) by itself, or HK$49.50 (about $6.40) with medium fries and drink. Easy to say, there probably isn't much truffle action going on for that price. Hong Kong-based blog Supersupergirl's Dining Reviews says the black truffle flavor wasn't strong enough to balance out the creaminess of the mayo.
According to the Wall Street Journal, the truffle burger promotion ends next month.