I would not use a microwave to prepare the butter caramel sauce you describe. Food temperatures are difficult to control inside a microwave and it's difficult to monitor the cooking process with any degree of accuracy looking through a glass covered shield with hundreds of little holes punched in it.
I suspect the recipe you describe contains error(s). My own experience involves meltin the butter into a saucepan over medium heat then adding the sugar and stirring until the sugar melts and begins to turn a soft brown color with an inviting aroma - before it is burnt. The amount of time I continue to cook it depends on whether I want a smooth sauce, soft ball or hard ball consistency. I usually get it off the heat sooner, rather than later, if I intend to use it as a sauce on cakes, cookies, etc. This sauce can turn from soft ball to hard ball stage in the blink of an eye and the only method I know of to maintain control is a candy thermometer or experience.