My girlfriend made the Joy of Cooking Classic Buttercream icing, which is sugar, water, cream of tartar, eggs and butter and is simmered.

The icing tastes right but icing was extremely greasy. She added powdered sugar, but the oils from the butter have formed greasy puddles and spots around the bottom of the cake. Does anyone know what caused the separation? This is a recipe we both use on a regular basis.

Thanks.