We make it with a Walmart cheapo food processor. Takes about 7 to 10 minutes of ear shattering racket. MY GF washes the buttermilk out but I don't bother. Leaves the flavour as far as I am concerned.
This method is easy and foolproof.
We tried shakin the jar to no avail.
We tried an electric, portable cake mixer. After about an hour we gave up. I made wooden paddles for it to see if that helped but no joy. We tried a manual egg beater, same thing, after awhile it seemed to be going nowhere.
For the record when the animals are outdoors eating hay in the summer there is a lot more butter.