Backstage, ET caught up with the adorable VanderWaal and judge Howie Mandel, who was an early believer in the young talent, where they both reacted to her stunning victory. VanderWaal already has plans on how she'll spend her $1 million grand prize, which will include a donation to a music charity.
"I want to go on [Animal Planet's] Treehouse Masters and get a sick treehouse -- zip lines, bridges, everything," she adorably revealed to ET.
VanderWaal recalled her winning moment, where she broke down and cried tears of joy onstage once her name was called. "Really whenever something like that happens, you're not really thinking. You're just like, 'Oh my gosh, oh my gosh, oh my gosh, this is happening, this is happening, this is happening, this is happening,'" she remembered thinking. "So it's really going through me."
Early in the season, Mandel used AGT's coveted golden buzzer on VanderWaal, a sign of supreme confidence.
"She is golden. She is [an] amazing grace. She is a shooting star. She is the brightest spot," a "proud" Mandel told ET. "I've been watching the show for 11 years, I've been doing it for seven, I have never been more excited and more thrilled to listen and anticipate every week's performance. I am such a big fan. If I was at home watching the show, I promise you, I would be downloading her tunes right away."
"I would be the first person to buy a ticket to her concert. I'll be the first person in the front row at Planet Hollywood in Vegas watching you," he told VanderWaal. "I cannot believe it. I am so excited to know you, to meet you, and to just be a small part of it."
"It feels really good," VanderWaal said when asked about Mandel's high praise. "Hopefully I will have like, a concert or something. If I ever do, you will also be the first one to be backstage." She also joked that she had requests for her hypothetical rider: M&Ms and Skittles!
On a more serious note, Mandel offered his advice for the young musician as she embarks on her new adventure.
"Just do what you do, be who you are and own it and continue to own it," he said. "Be yourself, don't be anybody else. Don't listen to anybody else, because what your heart has been telling you is has worked and gotten you this far."
VanderWaal admitted it still hasn't sunk in that she's the newest America's Got Talent winner.
"Absolutely not!" she said, before saying it out loud, as if to persuade herself it really actually did happen. "I just won America's Got Talent!"