Does anyone know if you can freeze a buttercream and white chocolate cake after it's decorated

Certainly :)  No problem.  Put it in a cake box and when you take it out of the fzr it's best if put into the frig overnight, then removed to the counter for a couple of hours.  Do NOT OPEN the box.  The cake will develop some sweating but that will evaporate and not be a problem if you do as stated above.