Getting friend zoned is... well I've never heard a girl say "I like you as a friend only." Girls are either interested in me romantically or not at all. And I think it's like that with all guys. All male/female relationships are sexual at the core. And I'm not great looking. I used to THINK I was in the friend zone, but then I took the risk of escalating. Held her hand, kissed her, etc.
My point is... If she's smiling at you, she's probably attracted to you already. You should talk to her like a normal person. You don't need to build attraction; you need to not LOSE the attraction that she already has for you. The way you lose attraction is by going into the situation thinking about how you can make her attracted to you. She either IS or she ISN'T. And you have no way to make her more attracted to you... Just like it would be hard for her to make you more attracted to her. You might be interested, but you would hardly care if she had a super interesting life or whatever. However, if she was to talk to you and stutter and be super shy and act with zero self-confidence, then you'd think something is wrong with her... You'd think that she wasn't good enough for you after all.
So... just be confident. You're good enough for her. That means that you should just talk to her, and invite her to hang out. You can say anything to her. You can say you think she's cute and you think you might want her in your life. You just have to say it confidently.
The only way to find out whether she's attracted to you is to assume that she is, and just do what you'd do if she was attracted to you. That's called confidence.