Then I give them a ticket and say "if you get stuck, let me know"
For the first little while, I am super responsive with them and answer everything immediately and eagerly.
After a while, I start to wait about 5-10 minutes before I respond to questions. Usually they figure out the answers before I even get back to them in these cases, and then I get to praise them for it.
I stumbled on it by chance when I failed to make it to a client meeting with one junior dev I was leading. He was counting on my presence to support him, we had already discussed it before. But I got caught up last minute and he decided to go alone. And I know it was hard for him, but everything went fine. And from then on he was a lot more confident in his day to day.