Boil brown sugar, granulated sugar, and milk until a small amount of syrup forms a very soft ball in cold water (232°F.). Add butter and remove from heat.

Cool to lukewarm (110°F.); beat until thick and creamy and of right consistency to spread.

If desired, place over hot water to keep soft while spreading.

Makes enough frosting to cover tops of two 9-inch layers, or tops and sides of two 8-inch layers.