From Rabbi Blog
This particular frosting goes with the Lemon Treasure Cake recipe, which at first seems like an odd combination, but it works really well. I did not have Nana's recipe card for this one and I looked around various websites until I found some common recipes. Below is the result and is tailored to be used with the Lemon Treasure Cake.
ROCKY MOUNTAIN FROSTING 1/2 cup butter 2 tbsp. all purpose flour 1/2 cup evaporated milk 1/2 cup firmly packed brown sugar 2 cups powdered sugar 1 tsp. vanilla extract 1 c. chopped walnuts Melt butter in a small saucepan over low heat; stir in flour. Add milk and cook until smooth and thickened. Remove from heat; add brown sugar and mix well. Add powdered sugar and vanilla, mixing well. Chop walnuts as needed to ensure they are manageable and mix into frosting. Cool; spread over cake Makes enough for top and sides of 1 (10") cake.