IMO. butter is safe to sit out for some time, but it doesn't hold its shape well when used for icing. I had a friend who decorated a doll cake every so carefully and woke up the next morning to a nude doll and and icing pooled on the table. Crisco is stable in all kinds of conditions. My recipe is similar to the one posted above, but had 3/4 cup of Crisco with no syrup, plus an egg white. I've cut the Crisco down slightly. I add a teaspoon of salt and almond flavor with the vanilla. It tastes wonderful and works up well. Sometimes I have to beat a little more powdered sugar after the cake is iced and before I do any decorating. The decorating icing has to be stiff if you want nice sharp edges.
White Decorator Icing
1 pound confectioner's sugar
1 egg white
1/2 cup + Crisco - no substitutions
2 T milk - you can add more, but you can't subtract it
1 teaspoon salt
1 1/4 teaspoons vanilla
3/4 teaspoon almond extract
Beat all together with a large mixer until very smooth, but stop before air holes start to develop. Add milk if too stiff, sugar if too thin.
I have a yummy frosting made with butter and marshmallow cream that held its shape in the heat, but the above one is the best for decorating.