AHello! First time poster here :-)

We live in a hot and humid part of Australia, and I'm wondering if I could make my daughters buttercream ruffle cake the day before her birthday and it still look good? I will be using a crusting buttercream to give it some hold, also how should it be stored overnight?

Thanks very much.

If you chill it immediately after you finish, it should set hard all the way through and remain in perfect condition.

Put the chilled cake into a sealed container BEFORE you leave it at room temperature.  This will prevent condensation. A cake box wrapped in a plastic bag will work fine.

If the crusting b'cream is the right consistency you should not need to do anything to hold it overnight.

I kept b'cream cakes at room temp for up to 3 days and never had a problem with them. 

It's best not to refrigerate cakes unless it has a perishable filling.  If necessary, then box it and place the box in a plastic bag (or wrap w/plastic cling film).