1Mix flour and milk until no lumps remain.
Cook on stove at medium-low stirring constantly, when it begins thickening turn stove to low and keep stirring until quite thick. Let cool a few minutes then put in refrigerator until cold.
2Place butter, shortening, sugar, vanilla and cold flour mixture in a bowl and beat until creamy and you taste no grains of sugar. (I use my kitchenaid stand mixer, much easier)
3This is the most delicious frosting, an absolute must on our family mayonnaise cake!