Step 1

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.

Step 2

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.