Correct temperature to serve a chocolate gateau (cake)
Most cakes should be served at room temperature, to maximize their flavor.  This one does not sound like an exception.  The ganache and buttercream should both hold well at room temperature on the day the cake is assembled (from its description, this is not a gateau that should be stored for a long time).

Don't take the common restaurant practice of serving cakes chilled as an indicator of good practice.   They do so because they don't want to store it at room temperature, and they usually wait to take it out of the refrigerator until the portion is ordered.