"at its heart" is a correct and usable phrase in written English. You can use it to express the most essential aspect of something. For example, "The novel is, at its heart, a story of courage and perseverance.".
At its heart, it is a nihilistic, psychological mystery.
At its heart it communicates in an international language.
"At its heart it is really about empathy.
Yet at its heart, it is so simple.
At its heart, it is about solidarity and social justice.
It was, at its heart, ugly playground comedy.
Whatever the mission is, it has news at its heart".
I love the desktop app, it’s always running on my Mac. Ludwig is the best English buddy, it answers my 100 queries per day and stays cool.
Cristina Valenza
Retail Lead Linguist @ Apple Inc.