Complex sentence: Custard tarts were introduced in Hong Kong in the 1940s by cha chaan tengs; then, they were introduced in western cafes and bakeries to compete with dim sum restaurants, particularly for yum cha.
Split into simpler sentences: Custard tarts were introduced in Hong Kong in the 1940s by cha chaan tengs. They were then introduced in western cafes and bakeries to compete with dim sum restaurants, particularly for yum cha.