the recipe needs 1/2 cup of butter and 1/2 cup shortening. if i use 1 cup of butter instead will it alter the results much?
It might be a little softer but the flavor will likely be better--just don't leave it directly in the sun and it'll be fine!
It will taste better. If you are trying to pipe or do other decorative work with it, it might be more difficult, as it can be softer/less stable than buttercream that includes shortening.