Commonly found in bakeries throughout the United States today, egg tarts or egg custard tarts first appeared in Hong Kong in the 1940s and originated from English custard cakes. A pastry-crust snack with egg custard and subsequently baked, snacking was popularized as an alternative and competition with dim sum restaurants which offered morning and afternoon snacks usually steamed and in small plates. Egg tarts come in two different forms. One has a flaky puff pastry base while the other a shortcrust butter pastry base. Good eggtarts maintain a buttery, soft, but firm and crumbly taste.
Sources: http://www.discoverhongkong.com/eng/dine-drink/what-to-eat/local-flavours/local-snacks.jsp, http://www.hongkongtraveladvisor.com/food/egg-tart, http://droolfactor.wordpress.com/2011/09/06/hk-style-egg-tarts/