Most of my buttercreams contain some cream cheese for a creamy smooth texture. And according to many articles, it can be left out overnight, although I recommend refrigerating your finished cakes until 2-3 hours before serving just so it doesn’t get too soft on your cakes. I’ll leave my cake leftovers that have cream cheese buttercream out for up to 24 hours because I want it to have a soft texture when I’m ready to eat it. The buttercream can stay in the fridge for up to a week, or you can freeze it for even longer! Just thaw it in the fridge overnight, and allow it to get to room temp before using.