There were some similar patterns on Ravelry, but they were all beyond my abilities or patience to increase said abilities. I boldly struck out on my own and I think it worked (and I wrote the details below in case you want to try, too)! But Phoebe keeps growing, so I'm not sure how long it will work.
How I made the shawl - also called a "sontag" in the 1800s
I started a dishcloth, which means I cast on 4 stitches. Knit all 4. Then slip one, knit one, yarn over, knit the rest of the row. Then repeat this (which adds a stitch every row while creating a picot edge) until you have 75 stitches.
Knit 25. Keep them on their needle and bind off the next 25. Leave the last 25 on their needle. With a third needle, work on one of the wings of 25 stitches. To continue the picot edge without adding any stitches, slip one, knit one, yarn over, knit two together, knit rest of row. Only do this when you are starting a row with the picot trim, otherwise just knit the whole row across. When the wing measures four inches, start decreasing: at the picot side, slip one, knit two together, yarn over, knit two together, knit rest of row. On the plain side, knit two together, knit rest of row. When you get down to 4 stitches, bind off. Repeat process for other wing. Weave in ends (there will be ends at the beginning of one wing, too, but I can't quite explain the details of how I got the yarn where I wanted it - trust me, it could not have been hard because as it must be obvious by now, I can't handle hard knitting).
Sew buttons on the wing tips. Wrap around baby with the wings going under her arms, to button on the point of the vee at the back. Adorable and warm.