Combine the room temperature salted and unsalted butter into a mixing bowl.
Using the handheld mixer, whisk or beat the butter until it is whipped. The butter will look lighter in color and take up more room in the mixing bowl.
Add in the vanilla and mix for a few seconds, or until fully incorporated.
Fold in about a 1/3 of the can of sweetened condensed milk and whip until incorporated. Then, repeat with each third of the sweetened condensed milk until all of it is added into the frosting.