Give the hardest part of the project to someone ready to grow into it, then stay close enough to catch them.

Growth happens on jobs slightly beyond what somebody has already done, which means the handover always feels a little risky. The two usual failures are opposite: giving the hard thing and then vanishing, or giving it and then hovering until it is really still yours. Close enough to catch is the narrow middle, where the person owns it and knows help exists. That balance is most of what developing anybody consists of.

Hand the difficult piece to whoever is nearly ready, and say plainly when you will check in. Let them make the recoverable mistakes. Be reachable at the moment it gets hard, not before.