Mined in Magic by Jenna Wolfhart

Each year, the chasm-dwelling dwarves compete in a series of trials to find the best among them. Chocolate-loving Astrid has never bothered to enter. She’d rather tend to her garden, bake moss cakes, or search the mines for a way to break her curse, which forever binds her to the mountains.
But someone else puts forth her name. She has no choice but to take part, especially when she learns the champion will win the Everstone—a gemstone with enough magic to break her curse.
At long last, Astrid’s freedom is within reach.
Of course, nothing is ever that simple. An infuriating shadow demon joins the competition, dead set on winning the Everstone himself. He’ll do whatever it takes to succeed, particularly if it involves pilfering Astrid’s cakes or tormenting her until she gives up hope.
But when the Everstone goes missing, they agree to join forces to track it down. And deep within the mines, they discover something in each other that is far more powerful than magic.