Who is the singer-songwriter (she's been on Letterman) who plays piano and has a little girl voice?

She's been on there in the last few months. She plays the piano and writes her own songs. If you get past concentrating on her funny sounding voice, she's very articulate, even profound, in her song lyrics, a lot like the great singer-songwriters of old like Janis Ian. But I can't find her name because there's no way to access a list of past guests on Letterman.