[pkallberg]

Macau, a former Portuguese colony, is full of these pastries. This may be how they entered China...

[Umofomia]

Indeed, they were popularized in Hong Kong via Macau:

    Custard tarts derived from the Portuguese pastry were
    introduced in Hong Kong in the 1940s by cha chaan tengs
    via the Portuguese colony of Macau.

https://en.wikipedia.org/wiki/Egg_tart#History