This recipe comes from my all-time life-long number one most beloved cookbook, The Fannie Farmer Cookbook. Fannie calls this Portsmouth Frosting and says, "this tastes of butter and cream, and what could be nicer?" What, indeed?
4 T melted butter
¼ C cream
1 tsp vanilla or rum*
About 3 C confectioners' sugar (I usually sift this)
Mix the butter, cream, and vanilla or rum together in a bowl. Slowly beat in the sugar until thick and creamy. Fannie says this is enough to fill and frost an 8- or 9-inch two-layer cake, but I often make 1½ times the recipe so I can lay it on extra thick.
* You can substitute another flavoring here, like almond extract or some kind of schnapps.