In a heavy saucepan, combine sugar with 1 cup water. Bring to a boil over high heat, swirling the pan occasionally to dissolve the sugar. Once the sugar has dissolved, boil the mixture, swirling occasionally (do not stir), until it has a medium amber color and honey-like consistency. (If the mixture seems to be browning too fast, lower heat to medium.) Remove pan from heat and stir in butter and salt. Slowly pour in goat's milk (mixture will bubble furiously) and stir until smooth. Let the cajeta cool slightly before serving.