In a bowl of a mixer fitted with a whisk attachment, whip butter until it has tripled in volume and is light and fluffy (approximately 8 minutes). Stop to scrape the bottom of the bowl a few times.
Add one third of the sweetened condensed milk while mixing on medium speed. Whip for about 8-10 seconds. Then, stop and scrape down the bowl. Repeat for the next third of the sweetened condensed milk and the final third. Stop mixing 8 seconds after adding the final third. Do not overmix.
It is VERY important to follow the recipe exactly as written. If you don't mix the butter long enough or don't add the sweetened condensed milk while mixing, the frosting may end up separated and too soupy. Different brands of sweetened condensed milk are different colors. I've found that Nestle is much lighter than Eagle Brand. Keep this in mind if you prefer a lighter looking frosting.This recipe frosts 12 cupcakes with a large swirl or 24 cupcakes with a small swirl.While this recipe is delicious as written, many people add salt, vanilla, or other flavorings to this frosting. It's a personal preference.