Brown -- who played Marnie Piper in the Disney franchise -- posted a cool TikTok video of herself, now at age 35, busting out a witch hat and plopping it on her head ...

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She writes, "Just wanted to see if it still fits...😏." As Kim finally makes the hat touch her noggin, the video cuts to her in the full "Halloweentown" witch costume, and the song changes too, of course.