How long can frosting with butter, powdered sugar, salt, vanilla, and milk, that is covered with fondant be left out of the refrigerator? I'm making a 9x13 chocolate cake, frosted with a vanilla buttercream, (not filled) and covered with black Fondarific fondant.

Here's the ingredients for the frosting:


1/2 cup unsalted butter
1-2 cups confectioners sugar
1/8 teaspoon salt
1/2 tablespoon vanilla
up to 2 tablespoons milk


The cake is going to be finished tonight, and eaten tomorrow afternoon. Is it safe to leave it out of the fridge?
Because of the dairy in your frosting, it really ought to be chilled overnight.  It could warm up on to room temperature for a few hours.  

Also, do cover the cake, so dust and stuff doesn't get on it.