For the recipe in question, the dry version of ladyfingers, which are interchangeable with savoiardi, should be used. The two can be used interchangeably in recipes that require a dry biscuit to be dipped into a sugar syrup or other flavored liquid, such as coffee when making a traditional tiramisu. Of course, the inconsistent use of the name as pertains to store-bought biscuits doesn’t help anyone, given that the term is also used for the already-soft, sponge-cake fingers commercially known as “ladyfingers.”) I make tiramisu and charlottes a lot, and find that using soft sponge cakes is a mistake most of the time.

In a recipe that calls for dipping into syrup or liqueur, the dry ones are better; they absorb a lot of liquid and become, therefore, like the sponge fingers, but with the flavor of whatever you’ve dipped them in. You really don’t want to soak them, but rather dip them. One second is often enough, or at most a second per side; you don’t want them to get too soggy.

I have actually made this recipe, and I think it’s grand. Hope you feel the same.

It can be tricky to use them interchangeably since ladyfingers don’t hold up as well as savoiardi to soaking. However, homemade versions of either can, of course, be made more or less dry and crisp, depending on proportions and baking time.

If you use savoiardi in this recipe, they would soften from the soaking and from the mousse. Ladyfingers would be much softer to begin with so you may want to leave them out overnight to dry out a bit before soaking them in this recipe. In any case, I don’t think they would fall apart or collapse because they’d adhere to the sides of the mousse as long as it has been well chilled before unmolding.