In top of a double boiler combine peanut butter and condensed milk. Cook over rapidly boiling water, stirring constantly, until thickened. Spread on your favorite cooled cake. Yields enough to cover tops and sides of two 8 inch cake layers or 12 cupcakes.
Note: Peanut Butter Coconut Frosting: Add 3/4 cup grated moist coconut at the end of the cooking. Then spread on cakes or cupcakes.