[Grammar] decorated with/by sugar

[atabitaraf]

There is a brand of biscuit, decorated with/by sugar.
Which preposition is correct?
I learned that 'by' is used for motor machines, humans or active agents.
Thanks for your help,

 

[Rover_KE]

with

 

[emsr2d2]

It might have been decorated with sugar by a person by a machine. I'm not even sure what an "active agent" is or how it would decorate anything!