cosmos is kind of leggy when they first start anyway. Just plant them out normally, they will be fine.
Duane

I planted out some leggy cosmos in the beds last week and they are doing fine now, all settled in and starting to put out more leaves. Just go for it.

You can definitely plant these deeper in the ground. Tomatoes also do well with this treatment. I also pinch my Cosmos to make it bushier - some of the cultivars of Cosmos bipinnatus get very tall and lean over if I don't do so (and sometimes even when I do).