Find some idea, get your friend on board as an investor / advisor ? You guys seem to get along well and have each others interests at heart otherwise you wouldn't say you loved the job that much.
Three birds with one stone, angel investor / mentor / continued professional relationship, only this time you are in charge.
I've loved all my jobs.
Also, don't discount all the data you get from your job to feed your startup plans.
I wouldn't dream of starting a competitor to the project we built together.