Melt the butter in a saucepan over low heat and let it brown slightly, not too dark, or just a hint of color. Use a wooden spoon to blend in the sugar and run until it is smooth. Stir in the water, only 1 teaspoon at a time, stopping once you reach a smooth pouring consistency. Immediately, but gently spread the hot glaze over the cake, smoothing it out quickly with a knife dipped in hot water. Some glaze may trickle down the sides, but the effect is quite decorative.