The Cantonese egg tart (蛋挞, dan tat) is a dim sum institution — a flaky or crumbly pastry shell filled with a silky, lightly set egg custard. The Hong Kong egg tart — associated particularly with Tai Cheong Bakery — is made with a crumbly water-and-lard shortcrust (水皮, shui pi) and a delicate, barely-set vanilla custard that is smooth, slightly wobbly, and never rubbery. It occupies the same territory as Portuguese pasteis de nata (from which it likely descends via Macau) but with a distinctly lighter, more yielding custard.
Two pastry types: (1) Flaky pastry (酥皮, su pi) — oil pastry laminated into water pastry. (2) Shortcrust (水皮, shui pi) — the more common HK style, made with plain flour, icing sugar, butter or lard, and egg. The shortcrust style is crumblier, lighter, and more delicate. The custard: 3 eggs + 1 egg yolk, 80g caster sugar, 200ml full cream milk, 100ml warm water, 1/2 tsp vanilla extract, a pinch of salt. Beat the eggs and sugar together without creating too much foam. Add milk, water, vanilla. Strain through a fine-mesh sieve. Pour into chilled raw pastry shells. Baking: 200C for 5 minutes to set the pastry base, then reduce to 160C and bake 15-20 minutes further. The custard should be just set at the edges with a slight wobble in the center.
Baking at too-high temperature throughout: The custard curdles and develops bubbles on its surface. Overmixing the custard: Beating the eggs vigorously incorporates air, producing a foam-topped tart rather than a silky-smooth surface.
Eileen Yin-Fei Lo, Mastering the Art of Chinese Cooking (2009); Ken Hom, Complete Chinese Cookbook (2011)