Ensure the butter is completely at room temperature before starting.
Place the butter in a stand mixer with the whisk attachment. Whip on high speed for 5 minutes, occasionally scraping down the sides of the bowl.
Reduce the mixer speed to medium. Add ⅓ of the condensed milk, whipping until fully blended. Repeat this step until all the condensed milk is incorporated.
Add vanilla and salt, then whip on high speed for 1 minute to finish.
