Why does this keep happening?

My Mother just seems to constantly get angry and throw a tantrum if she doesn't get things the way she wants. She never want to have a civil conversations about anything and all conversations with her constantly end in her yelling and running off while I remain calm. For example, I asked her for pop tarts as part of my Birthday present becuase I really don't get them much but she will never purchase these for me on any occasion and frowns if I buy themeself. She always has extremely over dramatic reactions to every disagreement. "fine, get the pop tarts, clog your arteries and die" This could not happen becuase of one serving of pop tarts. I know she is trying to keep me healthy but I eat quite healthy most of the time and I don't understand why I can't have my favorite foods in moderation. The problem is that I can't talk to her about this because she just says "enough, enough!" and yells and runs off. This happens so often and it's extremely frustrating. I am not the one yelling at her always seems to be getting overly angry and screaming . How can I talk to her about this?