A method of making a golf ball comprising the steps of providing an untreated golf ball core comprising a thermoset rubber composition, the untreated golf ball core comprising an untreated outer surface having a hardness; treating the untreated outer surface of the untreated golf ball core with a surface-softening material comprising at least one fatty acid or fatty acid salt composition to form a golf ball core comprising a treated outer surface having a hardness less than the hardness of the untreated outer surface; and forming at least one cover layer about the core to form the golf ball. Also, the untreated core may comprise a geometric center and an untreated outer surface, each having a hardness such that the hardness of the treated outer surface is less than both the hardness of the untreated geometric center and the hardness of the untreated outer surface.