Ok, so I posted up a thing a moment ago and then the internet decided to have a stroke and shut down websites selectively.
At this point, the mode of communication is irrelevant in terms of lameness. You're so emotionally invested now that no matter what manner of confession you use, it will come off as awkward and uncomfortable. The choice of mode here is not about impressing her.
And that's not that much of a problem in your case. You and her are already friends, so it's not about making a good impression. What this is about now is not succeeding in picking her up, because she already has a full formed opinion of you. It's about getting you to speak your mind, so as not to regret never saying anything for the rest of your life.
Consequently, the mode you choose being lame is almost unavoidable. Whether it's an uncomfortably awkward face-to-face chat, an unbearable phone call, or any of the electronic or other passive methods is irrelevant to your success. However, I stress, do not use a letter unless you're physically going to put it in her hands for her to read.
Anything that leaves a nonnegligible time period between you sending and you getting your reply is going to be AGONY. You are going to feel awful. For this reason, I cannot recommend an instantaneous medium more strongly.
Any social convention about "electronic relationship messages are bad", if it really exists (and I don't believe it does, outside of breakups), doesn't apply here because you already have a preexisting rapport. It's too late to make a first impression.
tl;dr: You're not trying to impress her, you're trying to get things off your chest. Use whatever medium you are most comfortable with, don't try and use one because you think it will look better. It is too late for that.