I used to make this years ago for another type of cake. It's similar to yours, but different amounts of ingredients. I think that I recall I had trouble with separation until I started using a good stick margarine instead of real butter.

FLOUR FROSTING

2 1/2 Tbsp. flour
1/2 cup milk
1/2 cup margarine
1/2 cup sugar
1 Tbsp. vanilla

Over heat in a small saucepan, blend the flour into the milk and cook to a thick paste. Let it cool completely.

In a separate bowl, cream the margarine with the sugar until fluffy. Then mix in the cold paste. Beat very well with mixer on medium or high speed. It will take a lot of beating because the paste of flour needs to get incorporated without being lumpy.

Add the vanilla, mix again and spread onto cooled cake. Hope this helps.

June