Yes. Believe it or not, it actually is that easy. And you should believe that she likes you, too. People don't go to formal events with people they don't like.
In addition to telling her that you like her -- which she already knows BTW (you *did* ask her to the formal, right?) -- tell her what you just told us. If you are able to find the words that let her know that talking to her (and only her) is incredibly easy, she will most likely be flattered beyond belief. It will make her feel very special -- and, in turn, she will probably think of you as very special. If you really have a connection with her, don't avoid cementing it. Just be really sensitive and not overbearing or aggressive, and I'm quite sure the message will be favorably received.
This happens to me all the time BTW. There's often that ONE SPECIAL GIRL that I can open up to -- while I keep the rest of the world at bay. I find it really hard to answer that question about whether I find it easy to express emotion. With that one "chosen one," no; with everyone else, yes.
Best of luck. You *deserve* this
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Put the curse of loneliness on every boy and every girl,
Until everybody's kickin', everybody's scratchin',
Everything seems to fail ?
And it was all for the want of a nail.