I've got an unopened box of powdered dreamwhip that the date expired last month. Should I throw it away or will it be ok to use in buttercream?

it's ok to use

but for clients i would replace it for an in date box -- but i would eat that and serve it to my family -- but if someone is paying i err on the side of psycho safe -- i envision being interviewed on my practices and methods and i always want to have a clear crisp guilt free report

It's for my daughters birthday cake. I don't sell, just bake for my kids. :)

you're fine -- no worries

I now teach high school cooking classes and I do a section on expiration dates.  Understand the diff between "sell by" and "use by".  Even the "use by" dates have time built in.  Many items (especially dry products) are still good 6 months or more after the expiration.  Manufacturers use the CYA method when setting dates on their products ...... they are going to cushion the date.