A Big Gulp plopped on top of a Steinway piano isn’t something I ever expected to see. But there it was, almost towering over Kristin Chenoweth, the petite performer with a powerhouse voice, in her enchanting Broadway concert debut, “My Love Letter to Broadway,” which plays through Nov. 13 at the Lunt-Fontanne Theater.

That staple of American culture — the Big Gulp, I mean — made for a nice reminder of Ms. Chenoweth’s appealingly earthy demeanor. You can take the girl out of Oklahoma, but you can’t take the Oklahoma out of the girl.

Which is just fine, because much of Ms. Chenoweth’s appeal is in her perky wholesomeness and golly-gee enthusiasm (not exclusive to the Sooner State, of course), which never feel manufactured. She has lost none of the girlish glow she possessed when she first rose to Broadway stardom, even as she made a winning, self-deprecating joke about her age (48), namely that she hasn’t yet appeared on the cover of the AARP magazine.

Nor does her singing ever feel mannered, although her bright lyric soprano is a superbly trained instrument. (Both she and Kelli O’Hara, also from Oklahoma, shared the same vocal teacher.) Throughout the concert — which lasted a generous two hours-plus, including intermission — her voice bloomed and soared almost effortlessly, in a wide range of repertoire, much but not all drawn from Broadway shows.