Soooo in my cake class on Tuesday I decorated a cake. The filling is ganache and the crumb coat with an outer layer of buttercream. I have had it in the fridge since would it keep until Sunday to serve for Canada Day?
I regularly keep cakes under refrigeration (single-layer with either cold-process buttercream or canned frosting) for 7-8 days.
Of course, your results may vary, depending on the exact type of buttercream. The cold-process recipe that's been on the back of the C&H powdered sugar box since before I was born, for example, has only four ingredients: powdered sugar, butter, milk, and vanilla extract. I use variations that might include strawberry jam, maple syrup, or cinnamon, but still, the ingredients, if fresh, will certainly keep.
Based on what I remember from when Alton Brown made it on Good Eats, ganache seems stable enough, but others here would be more qualified to give you a shelf-life under refrigeration. Just as they would be more qualified to give an opinion on the shelf-life of hot-process buttercreams, meringue buttercreams, and so forth.
The buttercream I use is powdered sugar, butter, shortening, vanilla extract, and whipping cream
For food safety, find your weakest link by product or product/process.
In this case it is the cream. By product, it must be refrigerated and will be good until the expiration on the package.
By product/process, the cream has had the preservative, powdered sugar, added to it in a large amount by ratio, making it much more safe, possibly even shelf stable. But since we don't have a lab, err on the side of caution to decide how to store.
For your cake, refrigerator is fine and the sugar will make it able to stay out at room temp on serving day.
And the whipping cream is most likely ultra-pasteurized, which might make it potentially a bit more stable than fresh, gently-pasteurized milk, although by no means as stable as shelf-stable milk (which is only shelf-stable so long as it remains in its hermetically-sealed container).
right hb, low temp pasteurization is much more volatile than ultra.
. . . which is why, up until recently, the only way to get certified-organic milk that wasn't ultra-pasteurized for long-distance shipment is to go directly to a farmer who's set up to pasteurize his own.
Thankfully, some months ago, I saw what appears to be gently-pasteurized certified-organic milk at either Bristol Farms, Sprouts, or Whole Foods (or maybe more than one of the above).
While I'd use ultra-pasteurized (and even shelf-stable) milk in cooking, it's not something I'd drink by choice. It tastes (and this goes doubly for the shelf-stable stuff) more like milk of magnesia than milk of a cow.
Who cares if it's certified organic, if it's been so heavily pasteurized as to be undrinkable?
Thanks for the great info, the cream is good until the end July. I decided to serve it to my daycare kids and my family tonight.... And I was super delicious, the whipped ganache was sooooo yummy!!