to OP: Do you want to give her a chance to change her mind or think a bit more about the possibility of dating you? Perhaps she had not really thought about it as much as you had when you had your "ultimatum" conversation. While I agree you should stand your ground on just being friends, do you really want to try to date her or are you already done? I would act like yourself and be friendly with her, but I would reiterate your desire to be more than friends. She may just need a little time to change her thinking of you as a friend to as a potential boyfriend. Being friendly and confident may change her mind but being distant and pouting will ensure she does not want to date you.
I dated my best friend from grad school, thought we had so much in common. Well we did have a lot in common and liked the same things, liked spending time together, etc. We spent 4 good years together before we mutually agreed to break up when it became clear we had different ideas about the future. To make a long story short, I totally lost my best friend. I was glad to not be dating him any more and felt good about moving on from him as a boyfriend, but I felt as if my best friend died when we broke up. It's true there is no being friends after you break up. I think we spent the last couple years staying together because we each did not want to lose our best friend. I was profoundly sad that he was not in my life anymore. We have had a couple awkward lunches since then and sometimes we fall into a sense of comfortable conversation, but on the large part we have spoken only a few times in the last few years. I have moved on and am totally in love with a great man now, but I guess I regret that I don't really know my "best friend" anymore and probably never will again.