But it’s really a personal decision. Apparently because of all the sugar you can leave a cake with buttercream frosting out for 2-3 days. How long can you leave a cake with buttercream icing out? I ...
If it doesn't, you can grab extra powdered sugar and start slowly ... just about ½ ounce per cup of frosting — keep this amount low if you're making a buttercream frosting.
To make the buttercream icing, beat the butter until soft, then sift in the icing sugar and cocoa. Add the milk and combine together until soft. Spread half the icing over the cake up to the edges.