Not all buttercreams are created equal. Here’s a quick cheat sheet so you don’t poison your guests.
American Buttercream (butter + powdered sugar + vanilla): Counter-safe for a day or two. No worries.
Swiss Meringue Buttercream (egg whites + sugar + butter): Needs a little more caution. Because of the cooked egg whites, it’s safer to refrigerate if you’re not eating it within 12 hours. But it’s still fairly stable.
Italian Meringue Buttercream (hot sugar syrup + egg whites + butter): Same deal as Swiss. Better safe than sorry—fridge it.
Does Buttercream Thicken In Fridge at Bailey Walters blog
Cream Cheese Buttercream (cream cheese + butter + sugar): Fridge required! Cream cheese is not your counter-friendly friend. It spoils quickly and can turn sour. Do not mess with cream cheese.
A Note on Fondant vs. Buttercream
Fondant cakes (those smooth, shiny ones) are usually fine at room temp because fondant is basically sugar play-dough. But if they have buttercream underneath? Same rules apply.
The frosting type matters, but the filling matters more. A cake with a raspberry jam filling is fine on the counter. A cake with a Bavarian cream filling needs to be chilled like you’re storing precious jewels.
