Whip room temp butter in a stand mixer till smooth and light (about 5 minutes). It should loose it's yellow color.
250 grams butter, room temp
Weigh your powdered sugar in a separate bowl. Once the butter is whipped and light in color, sift in powdered sugar one cup at a time. Whip sugar in very well before adding the next cup. Add milk 1 teaspoon at a time, till you have the consistency you desire. Whip the frosting well till light and fluffy.