In small bowl, beat 1/4 cup butter and the shortening on medium speed until light and fluffy. Beat in vanilla and dash salt. On low speed, gradually beat in powdered sugar until smooth. On high speed, beat in 1 tablespoon milk until light and fluffy. Add enough additional milk until frosting can be spread or piped. Frost or decorate cookies as desired. Let stand until frosting is set, about 30 minutes. Store in loosely covered container.