Why did the witch crack like porcelain?

Please note: most of my answer is based on supposition and experience as a Director.

There are no references as far as I can tell (under cursory research) about superstitions related to the facade of a witch cracking under duress or to any similar behavior in the "Dark Shadows" original series.

It is assumable that the Director (Burton) does this for cinematic and symbolic reasons.

A witch (often like a vampire) has a lot of mythological license (along the lines of poetic license). The Wicked Witch of the West melts from water, Baba Yaga is weakened if her mirror is destroyed. Witch is also a generic term that can range from "Oz" to "Practical Magic" to "Buffy." It can reference a supernatural creature or a normal, human woman who uses supernatural forces.

So, the penalties or side effects can be as varied.

So, focus on the character and the effect. "The beautiful woman cracks when angry showing a hollow interior"

Honestly, this is "Directing Symbolism" 304. (Third year, major class, because you want some terminology and experience before you start doing in depth analysis). The idea of a beautiful veneer cracking can be symbolic of the beauty being damaged. A veneer can be a false cover. The hollow interior... One can run with that about as far as one can run with Tony Stark having no heart.

I don't think there is a specific "Canonical" reason for it. I think we can basically point to Burton making a directorial decision. And I'd hazard a successful one if it resulted in someone asking, "Why"

From an interview with Eva Green (who played Angelique):

I’ve always been a fan of Tim’s,” Green says. “He’s so creative, but also open to suggestions, which is wonderful for an actor. We had the same understanding of Angelique’s character. He never treated her like a one-dimensional villain; he got her pain.”

Angelique is a woman who has changed with the times. During the 18th century, Angelique was a dark-haired servant girl. As Angie, the CEO of Angel Bay, she’s a successful blonde businesswoman. “Tim wanted her to look like the American dream,” says Green. “Everything about her is perfect. Too perfect. Perfect makeup, red lips, platinum hair. She’s very glamorous yet sophisticated. But, little by little, from the moment Barnabas escapes from his tomb, her facade starts to crack.”

This article explains the effect they were after.

It is rather interesting. In the Brothers' Grimm, starring Matt Damon and Heath Ledger, the queen/witch ends up cracking, too, when her vanity mirror is smashed- she falls away just like her mirror. So when I saw Dark Shadows, I immediately thought of this movie, and I was curious as to why witches are viewed so differently in every tale. They always have a different aspect- some are immortal, some have eternal youth, some melt in water, some need to use spells, others just wave their hands, some are made of glass, some use wands, some ride on brooms, others just fly.... The list goes on. So I agree that there is a mythological license to be had when it comes to witches, and I think Burton did a brilliant job. In order to expose Angelique as a witch, Collins had to crack her neck, and show that she didn't die, but was instead made of glass and a hollow exterior. It was a very dramatic moment in the movie, and worked rather well to build up to the ultimate fight they had. It also supported the idea that she didn't have a heart- until she pulled it out and watched it shatter as she died.

My only question to add to this is that if she were hollow and made of glass, how is it that Collins didn't recognize it right away when he met her? Would her flesh really have been warm like a human's? Would she have a pulse? If one knocked on her, would she sound hollow? It doesn't really matter to the overall plot, but I am rather curious. :)