What procedure would fix my chin/jawline ("witch's chin")? (photo)

The horizontal line is known as a mental crease- this tells me you have an active mentalis muscle. Botox in the mentalis would help with the protrusion you complain of as well as the mental crease.
Once Botox is on board, using HA fillers in the chin can give you a more heart shaped face and reduce the "butt chin"

Hi, I’m Lauren, the aesthetic injector here at Colorado Plastic Surgery Center.
A great nonsurgical option to help you achieve your goals would be dermal filler treatments. Filler can be used to contour the chin and jawline, creating a more elongated appearance and helping to achieve the balanced, V-shaped look you’re describing. If jaw width is a concern, Masseter Botox can be used to create a slimming effect over time, which further enhances that soft, feminine V-line. It's always a good idea to consult a licensed provider for a personalized treatment plan based on your needs.

I suspect the most reliable way of addressing your concerns is with an injectable filler. I do not believe that you are a candidate for chin implant surgery, since your chin is over projected as is. If you would be satisfied with filling in the indentation, depending on your examination, microdroplet Silikon-1000 injections may be considered for a permanent result. Please consider meeting with several reputable Facial plastic surgeons. Thank you for this question. Since the only Dr Joseph