you can try an urschel mill. note almost all mills generate heat when grinding. grind slowly to generate less heat. or add dry ice.
I only did this once, but actually had very good luck using a robocoupe with a shredding disc, the sort normally used for making things like shredded vegetables.
The shreds break up into a relatively fine (if irregular) powder.
I was planning to use this as seed for tempering, but then found out that I could use mycryo, so I did not develop the process past one attempt.