work after one's heart; one's masterpiece
A set phrase meaning a work that fully satisfies its creator; often used for artistic or literary works.
This painting is his work after his own heart.
I'm very happy to have created a work that satisfies me.
From 会心 (satisfaction, gratification) + の + 作 (work). The exact historical derivation is uncertain, but the phrase is conventionally associated with a creator's personal satisfaction.