So no black cochin rooster?
The fastest would be to have both Cochin hens and a rooster. Use the chocolate rooster once then discard and continue with the offspring. Using offspring back to cochin every generation.
Color will be easy to carry forwards but type should be the focus for a while.
To...
Chocolate is sex linked so a female only carries one gene. Either chocolate or not chocolate. She can't carry the gene unseen so if she has it it shows.
Males have two genes so they can have 2 not chocolate, 2 chocolate (those will show chocolate) or 1 chocolate and 1 not chocolate. Those will...