Fairytale about a boy who goes to steal a princess' golden heart and replaces it with a rotten apple

I had this story read to me as a child, and I only remember vague parts of it. It would be pre-2000.

There was a princess with a heart of gold, and a boy who set out to steal it (literally). On his way to the kingdom, he encounters a witch who teaches him how to cut someone open and stitch them up without a trace. She does this by grabbing his arm and slashing it with a knife, then pulling out a piece of silk exactly the same colour as his skin and stitching it over the wound so it was as good as new. I always remembered that bit, because it lowkey horrified me as a child.

When he gets to the castle, he sneaks into the princess' room and cuts her open. He has a porcelain heart which he intends to swap with her golden heart, but he accidentally breaks it. Panicking, he grabs the nearest thing, which was an apple, and replaces her heart with it and sews her back up.

Over the next few weeks, the princess gets more and more unpleasant, until the boy knows something is very wrong, and he sneaks back into the castle. He cuts open the sleeping princess again, and discovers there was a worm in the apple and it is slowing eating it up.

I don't remember the rest of the story, but I would like to find it. I'm not sure if it was a short fairytale, or a novel .