Put the whisk attachment on your mixer and in the bowl, add the butter, whipping it for approximately 5-7 minutes, until it’s airy and fluffy. Scrape down the bowl as necessary.
Add the vanilla and combine.
Turn off the mixer and add a third of the condensed milk, turn the mixer back on and slowly ramp up the speed to whip for about 10-15 seconds. Add another third, repeat, then add the remaining third and whip the contents for about 2-5 minutes or until the consistency is frosting-like.
Spread immediately or chill for about 10-15 minutes to firm the frosting as it will be quite soft.
This recipe makes more than needed for 12 cupcakes, probably more like 18.