Margarine or unsalted butter can be used instead of Crisco shortening to create cake frosting that is made with powdered sugar. However, making a substitution does slightly affect the taste and consistency of the finished product.
It is best not to use salted butter because the salt can dramatically change the taste of the frosting. Both unsalted butter and margarine can be swapped in the same volume as the shortening. However, using half shortening and half of either unsalted butter or margarine returns the best results. This is especially important if the finished cake is going to experience heat. Neither butter nor margarine stand up well to heat. Shortening fares better in heat because it adds texture and stability.