Buttercream frosting with 2T buttermilk - refrigerate overnight or no? Typically I leave my buttercream-frosted cakes out of the refrigerator. However, tonight I tried a new recipe that called for adding 1 tbsp lemon juice and 2 tbsp buttermilk (in addition to 1 cup butter and ~4 cups powdered sugar). Does the inclusion of cultured buttermilk warrant a night in the fridge, or is it safe on the counter? I plan on serving these in about 10 hours.

FYI: the result is a frosting that tastes like cream cheese frosting, but with a far creamier texture!
Considering the extremely high concentration of sugar in this style of frosting, it should be fine stored at room temperature as you usually do. 

The concentration of sugar will actually help dehydrate and kill those live cultures present in both the buttermilk and lemon juice through osmosis.