Put water and butter in a large microwaveable dish and let it come to a boil in your microwave; about 45-60 seconds. Keep an eye on it.
Add HOT water/butter mixture to mixer and blend until sugar dissolves. Scrape the sides if you need to and blend again. For best results, cool completely before using in dessert recipes. (The sweetened condensed milk will thicken more as it cools.)