Hello-

I received a cake order & the customer requested a french vanilla buttercream as a filling which is made with egg yolks.  I usually fill & stack my cakes 2 days prior & decorate one day before.  How long can this buttecream stay out?  Concerned about it going bad since it has egg yolks.  I'm used to using American buttercream which can be left out in room temperature.

-Thank you in advance!

Honestly, I think you are going to have to be in and out of the fridge with this one. The eggy cream, although cooked, is a bit dangerous to leave at room temp for 2 or 3 days plus the duration of the event. 

Ayes hazardous when it has been above 40 degrees Fahrenheit for more than 4 hours total after cooking it -- so needs to be brought to 40 within 6 hours of cooking it --

then say it's out of the fridge for 30 mins while you assemble/fill the cake -- you get it back in the box to chill down - take it out later to final ice for 30 mins -- back in the box -- take it out to decorate for an hour -- you got 2 hours left before you are serving hazardous food-- not that it tastes bad but the bacteria has been warmed long enough that it is multiplying too fast and furious now to be safe for all to eat --

Do you have to use the egg version of French Vanilla Buttercream?  If it's just the flavour she wants, there's lots of recipes out there for french vanilla flavoured buttercream without eggs.  If it's that very specific recipe she's requested, then I would have the same concerns as above.